Crash when trying to open a non-existing document

[markusressel]

Describe the bug
When trying to open a non-existent document, the app crashes.

To Reproduce
This requires a "out-of-sync" file tree in the app. It is then possible to tap on a document that exists in the app, but not on the backend.

Expected behavior
Close the editor (or display offline version if possible?) and display some kind of helpful error message.