[web] avoid race condition while deleting database

Summary: In previous code it was possible to schedule ops on not initialized DB.

Test Plan:

1. Add sleep after this line.
2. Trigger database deletion and right after schedule some ops.
3. Make sure there is no Database not initialized error.

Reviewers: tomek, michal

Reviewed By: tomek, michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10306