diff --git a/web/modals/threads/thread-settings-modal.react.js b/web/modals/threads/thread-settings-modal.react.js
index c5a0f8198..861223f5f 100644
--- a/web/modals/threads/thread-settings-modal.react.js
+++ b/web/modals/threads/thread-settings-modal.react.js
@@ -1,570 +1,579 @@
 // @flow
 
 import classNames from 'classnames';
 import invariant from 'invariant';
 import _pickBy from 'lodash/fp/pickBy';
 import * as React from 'react';
 
 import {
   deleteThreadActionTypes,
   deleteThread,
   changeThreadSettingsActionTypes,
   changeThreadSettings,
 } from 'lib/actions/thread-actions';
 import { createLoadingStatusSelector } from 'lib/selectors/loading-selectors';
 import { threadInfoSelector } from 'lib/selectors/thread-selectors';
 import {
   threadHasPermission,
   threadTypeDescriptions,
   robotextName,
 } from 'lib/shared/thread-utils';
 import {
   type ThreadInfo,
   threadTypes,
   assertThreadType,
   type ChangeThreadSettingsPayload,
   type UpdateThreadRequest,
   type LeaveThreadPayload,
   threadPermissions,
   type ThreadChanges,
 } from 'lib/types/thread-types';
 import type { UserInfos } from 'lib/types/user-types';
 import {
   useDispatchActionPromise,
   useServerCall,
   type DispatchActionPromise,
 } from 'lib/utils/action-utils';
 import { firstLine } from 'lib/utils/string-utils';
 
 import { useSelector } from '../../redux/redux-utils';
 import css from '../../style.css';
 import Modal from '../modal.react';
 import ColorPicker from './color-picker.react';
 
 type TabType = 'general' | 'privacy' | 'delete';
 type TabProps = {|
   +name: string,
   +tabType: TabType,
   +selected: boolean,
   +onClick: (tabType: TabType) => void,
 |};
 class Tab extends React.PureComponent<TabProps> {
   render() {
     const classNamesForTab = classNames({
       [css['current-tab']]: this.props.selected,
       [css['delete-tab']]:
         this.props.selected && this.props.tabType === 'delete',
     });
     return (
       <li className={classNamesForTab} onClick={this.onClick}>
         <a>{this.props.name}</a>
       </li>
     );
   }
 
   onClick = () => {
     return this.props.onClick(this.props.tabType);
   };
 }
 
 type BaseProps = {|
   +threadID: string,
   +onClose: () => void,
 |};
 type Props = {|
   ...BaseProps,
   +threadInfo: ThreadInfo,
   +changeInProgress: boolean,
   +viewerID: ?string,
   +userInfos: UserInfos,
   +dispatchActionPromise: DispatchActionPromise,
   +deleteThread: (
     threadID: string,
     currentAccountPassword: string,
   ) => Promise<LeaveThreadPayload>,
   +changeThreadSettings: (
     update: UpdateThreadRequest,
   ) => Promise<ChangeThreadSettingsPayload>,
 |};
 type State = {|
   +queuedChanges: ThreadChanges,
   +errorMessage: string,
   +accountPassword: string,
   +currentTabType: TabType,
 |};
 class ThreadSettingsModal extends React.PureComponent<Props, State> {
   nameInput: ?HTMLInputElement;
   newThreadPasswordInput: ?HTMLInputElement;
   accountPasswordInput: ?HTMLInputElement;
 
   constructor(props: Props) {
     super(props);
     this.state = {
       queuedChanges: Object.freeze({}),
       errorMessage: '',
       accountPassword: '',
       currentTabType: 'general',
     };
   }
 
   componentDidMount() {
     invariant(this.nameInput, 'nameInput ref unset');
     this.nameInput.focus();
   }
 
   componentDidUpdate(prevProps: Props) {
     if (this.state.currentTabType !== 'delete') {
       return;
     }
 
     const permissionForDeleteTab = this.hasPermissionForTab(
       this.props.threadInfo,
       'delete',
     );
     const prevPermissionForDeleteTab = this.hasPermissionForTab(
       prevProps.threadInfo,
       'delete',
     );
 
     if (!permissionForDeleteTab && prevPermissionForDeleteTab) {
       this.setTab('general');
     }
   }
 
   hasPermissionForTab(threadInfo: ThreadInfo, tab: TabType) {
     if (tab === 'general') {
       return threadHasPermission(threadInfo, threadPermissions.EDIT_THREAD);
     } else if (tab === 'privacy') {
       return threadHasPermission(
         threadInfo,
         threadPermissions.EDIT_PERMISSIONS,
       );
     } else if (tab === 'delete') {
       return threadHasPermission(threadInfo, threadPermissions.DELETE_THREAD);
     }
     invariant(false, `invalid tab ${tab}`);
   }
 
   possiblyChangedValue(key: string) {
     const valueChanged =
       this.state.queuedChanges[key] !== null &&
       this.state.queuedChanges[key] !== undefined;
     return valueChanged
       ? this.state.queuedChanges[key]
       : this.props.threadInfo[key];
   }
 
   namePlaceholder() {
     return robotextName(
       this.props.threadInfo,
       this.props.viewerID,
       this.props.userInfos,
     );
   }
 
   changeQueued() {
     return (
       Object.keys(
         _pickBy(
           (value) => value !== null && value !== undefined,
           // the lodash/fp libdef coerces the returned object's properties to the
           // same type, which means it only works for object-as-maps $FlowFixMe
         )(this.state.queuedChanges),
       ).length > 0
     );
   }
 
   render() {
     const inputDisabled =
       this.props.changeInProgress ||
       !this.hasPermissionForTab(
         this.props.threadInfo,
         this.state.currentTabType,
       );
 
     let mainContent = null;
     if (this.state.currentTabType === 'general') {
       mainContent = (
         <div>
           <div>
             <div className={css['form-title']}>Thread name</div>
             <div className={css['form-content']}>
               <input
                 type="text"
                 value={firstLine(this.possiblyChangedValue('name'))}
                 placeholder={this.namePlaceholder()}
                 onChange={this.onChangeName}
                 disabled={inputDisabled}
                 ref={this.nameInputRef}
               />
             </div>
           </div>
           <div className={css['form-textarea-container']}>
             <div className={css['form-title']}>Description</div>
             <div className={css['form-content']}>
               <textarea
                 value={this.possiblyChangedValue('description')}
                 placeholder="Thread description"
                 onChange={this.onChangeDescription}
                 disabled={inputDisabled}
               ></textarea>
             </div>
           </div>
           <div className={css['edit-thread-color-container']}>
             <div className={`${css['form-title']} ${css['color-title']}`}>
               Color
             </div>
             <div className={css['form-content']}>
               <ColorPicker
                 id="edit-thread-color"
                 value={this.possiblyChangedValue('color')}
                 disabled={inputDisabled}
                 onChange={this.onChangeColor}
               />
             </div>
           </div>
         </div>
       );
     } else if (this.state.currentTabType === 'privacy') {
       mainContent = (
         <div className={css['edit-thread-privacy-container']}>
           <div className={css['modal-radio-selector']}>
             <div className={css['form-title']}>Thread type</div>
             <div className={css['form-enum-selector']}>
               <div className={css['form-enum-container']}>
                 <input
                   type="radio"
                   name="edit-thread-type"
                   id="edit-thread-open"
                   value={threadTypes.CHAT_NESTED_OPEN}
                   checked={
                     this.possiblyChangedValue('type') ===
                     threadTypes.CHAT_NESTED_OPEN
                   }
                   onChange={this.onChangeThreadType}
                   disabled={inputDisabled}
                 />
                 <div className={css['form-enum-option']}>
                   <label htmlFor="edit-thread-open">
                     Open
                     <span className={css['form-enum-description']}>
                       {threadTypeDescriptions[threadTypes.CHAT_NESTED_OPEN]}
                     </span>
                   </label>
                 </div>
               </div>
               <div className={css['form-enum-container']}>
                 <input
                   type="radio"
                   name="edit-thread-type"
                   id="edit-thread-closed"
                   value={threadTypes.CHAT_SECRET}
                   checked={
                     this.possiblyChangedValue('type') ===
                     threadTypes.CHAT_SECRET
                   }
                   onChange={this.onChangeThreadType}
                   disabled={inputDisabled}
                 />
                 <div className={css['form-enum-option']}>
                   <label htmlFor="edit-thread-closed">
                     Secret
                     <span className={css['form-enum-description']}>
                       {threadTypeDescriptions[threadTypes.CHAT_SECRET]}
                     </span>
                   </label>
                 </div>
               </div>
             </div>
           </div>
         </div>
       );
     } else if (this.state.currentTabType === 'delete') {
       mainContent = (
         <>
           <div>
             <p className={css['italic']}>
               Your thread will be permanently deleted. There is no way to
               reverse this.
             </p>
           </div>
           <div className={css['edit-thread-account-password']}>
             <p className={css['confirm-account-password']}>
               Please enter your account password to confirm your identity
             </p>
             <div className={css['form-title']}>Account password</div>
             <div className={css['form-content']}>
               <input
                 type="password"
                 placeholder="Personal account password"
                 value={this.state.accountPassword}
                 onChange={this.onChangeAccountPassword}
                 disabled={inputDisabled}
                 ref={this.accountPasswordInputRef}
               />
             </div>
           </div>
         </>
       );
     }
 
     let buttons = null;
     if (this.state.currentTabType === 'delete') {
       buttons = (
         <input
           type="submit"
           value="Delete"
           onClick={this.onDelete}
           disabled={inputDisabled}
         />
       );
     } else {
       buttons = (
         <input
           type="submit"
           value="Save"
           onClick={this.onSubmit}
           disabled={inputDisabled || !this.changeQueued()}
         />
       );
     }
 
     const tabs = [
       <Tab
         name="General"
         tabType="general"
         onClick={this.setTab}
         selected={this.state.currentTabType === 'general'}
         key="general"
       />,
     ];
     if (this.possiblyChangedValue('parentThreadID')) {
       tabs.push(
         <Tab
           name="Privacy"
           tabType="privacy"
           onClick={this.setTab}
           selected={this.state.currentTabType === 'privacy'}
           key="privacy"
         />,
       );
     }
     const canDeleteThread = this.hasPermissionForTab(
       this.props.threadInfo,
       'delete',
     );
     if (canDeleteThread) {
       tabs.push(
         <Tab
           name="Delete"
           tabType="delete"
           onClick={this.setTab}
           selected={this.state.currentTabType === 'delete'}
           key="delete"
         />,
       );
     }
 
     return (
       <Modal name="Thread settings" onClose={this.props.onClose} size="large">
         <ul className={css['tab-panel']}>{tabs}</ul>
         <div className={css['modal-body']}>
           <form method="POST">
             {mainContent}
             <div className={css['form-footer']}>
               {buttons}
               <div className={css['modal-form-error']}>
                 {this.state.errorMessage}
               </div>
             </div>
           </form>
         </div>
       </Modal>
     );
   }
 
   setTab = (tabType: TabType) => {
     this.setState({ currentTabType: tabType });
   };
 
   nameInputRef = (nameInput: ?HTMLInputElement) => {
     this.nameInput = nameInput;
   };
 
   newThreadPasswordInputRef = (newThreadPasswordInput: ?HTMLInputElement) => {
     this.newThreadPasswordInput = newThreadPasswordInput;
   };
 
   accountPasswordInputRef = (accountPasswordInput: ?HTMLInputElement) => {
     this.accountPasswordInput = accountPasswordInput;
   };
 
   onChangeName = (event: SyntheticEvent<HTMLInputElement>) => {
     const target = event.currentTarget;
     const newValue =
       target.value !== this.props.threadInfo.name ? target.value : undefined;
     this.setState((prevState: State) => ({
       ...prevState,
       queuedChanges: {
         ...prevState.queuedChanges,
         name: firstLine(newValue),
       },
     }));
   };
 
   onChangeDescription = (event: SyntheticEvent<HTMLTextAreaElement>) => {
     const target = event.currentTarget;
     const newValue =
       target.value !== this.props.threadInfo.description
         ? target.value
         : undefined;
     this.setState((prevState: State) => ({
       ...prevState,
       queuedChanges: {
         ...prevState.queuedChanges,
         description: newValue,
       },
     }));
   };
 
   onChangeColor = (color: string) => {
     const newValue = color !== this.props.threadInfo.color ? color : undefined;
     this.setState((prevState: State) => ({
       ...prevState,
       queuedChanges: {
         ...prevState.queuedChanges,
         color: newValue,
       },
     }));
   };
 
   onChangeThreadType = (event: SyntheticEvent<HTMLInputElement>) => {
     const uiValue = assertThreadType(parseInt(event.currentTarget.value, 10));
     const newValue =
       uiValue !== this.props.threadInfo.type ? uiValue : undefined;
     this.setState((prevState: State) => ({
       ...prevState,
       queuedChanges: {
         ...prevState.queuedChanges,
         type: newValue,
       },
     }));
   };
 
   onChangeAccountPassword = (event: SyntheticEvent<HTMLInputElement>) => {
     const target = event.currentTarget;
     this.setState({ accountPassword: target.value });
   };
 
   onSubmit = (event: SyntheticEvent<HTMLInputElement>) => {
     event.preventDefault();
     this.props.dispatchActionPromise(
       changeThreadSettingsActionTypes,
       this.changeThreadSettingsAction(),
     );
   };
 
   async changeThreadSettingsAction() {
     try {
       const response = await this.props.changeThreadSettings({
         threadID: this.props.threadInfo.id,
         changes: this.state.queuedChanges,
       });
       this.props.onClose();
       return response;
     } catch (e) {
       this.setState(
         (prevState) => ({
           ...prevState,
           queuedChanges: Object.freeze({}),
           accountPassword: '',
           errorMessage: 'unknown error',
           currentTabType: 'general',
         }),
         () => {
           invariant(this.nameInput, 'nameInput ref unset');
           this.nameInput.focus();
         },
       );
       throw e;
     }
   }
 
   onDelete = (event: SyntheticEvent<HTMLInputElement>) => {
     event.preventDefault();
     this.props.dispatchActionPromise(
       deleteThreadActionTypes,
       this.deleteThreadAction(),
     );
   };
 
   async deleteThreadAction() {
     try {
       const response = await this.props.deleteThread(
         this.props.threadInfo.id,
         this.state.accountPassword,
       );
       this.props.onClose();
       return response;
     } catch (e) {
       const errorMessage =
         e.message === 'invalid_credentials'
           ? 'wrong password'
           : 'unknown error';
       this.setState(
         {
           accountPassword: '',
           errorMessage: errorMessage,
         },
         () => {
           invariant(
             this.accountPasswordInput,
             'accountPasswordInput ref unset',
           );
           this.accountPasswordInput.focus();
         },
       );
       throw e;
     }
   }
 }
 
 const deleteThreadLoadingStatusSelector = createLoadingStatusSelector(
   deleteThreadActionTypes,
 );
 const changeThreadSettingsLoadingStatusSelector = createLoadingStatusSelector(
   changeThreadSettingsActionTypes,
 );
 
 export default React.memo<BaseProps>(function ConnectedThreadSettingsModal(
   props: BaseProps,
 ) {
   const changeInProgress = useSelector(
     (state) =>
       deleteThreadLoadingStatusSelector(state) === 'loading' ||
       changeThreadSettingsLoadingStatusSelector(state) === 'loading',
   );
   const viewerID = useSelector(
     (state) => state.currentUserInfo && state.currentUserInfo.id,
   );
   const userInfos = useSelector((state) => state.userStore.userInfos);
   const callDeleteThread = useServerCall(deleteThread);
   const callChangeThreadSettings = useServerCall(changeThreadSettings);
   const dispatchActionPromise = useDispatchActionPromise();
-  const threadInfo = useSelector(
+  const threadInfo: ?ThreadInfo = useSelector(
     (state) => threadInfoSelector(state)[props.threadID],
   );
+  if (!threadInfo) {
+    return (
+      <Modal onClose={props.onClose} name="Invalid thread">
+        <div className={css['modal-body']}>
+          <p>You no longer have permission to view this thread</p>
+        </div>
+      </Modal>
+    );
+  }
 
   return (
     <ThreadSettingsModal
       {...props}
       threadInfo={threadInfo}
       changeInProgress={changeInProgress}
       viewerID={viewerID}
       userInfos={userInfos}
       deleteThread={callDeleteThread}
       changeThreadSettings={callChangeThreadSettings}
       dispatchActionPromise={dispatchActionPromise}
     />
   );
 });