Why are we relaxing the types here?

In the createSidebarSpec we're calling createMessageInfo, which can return any message type. The limitation of getMessageTitle seemed quite arbitrary to accept all the message types but the sidebar source. So I thought that widening the type here might be a good idea. But after a second thought, it doesn't seem like the best solution. Even though we're accepting the sidebar source here, it would still cause an invariant to be called in the sidebarSourceMessageSpec. So it's better to add an invariant sourceMessageInfo.type !== messageTypes.SIDEBAR_SOURCE to createSidebarSpec.