Crash when trying to open a non-existing document

**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.
