Fix crash when receiving remote change in unfocused editor

[mirka]

This fixes an uncaught bug in #1193, where we get a WSOD when the selected note receives a remote change while the editor is unfocused.

This was caused by the same strange behavior we worked around in getCurrentBlock(), where the selectionState methods getAnchorKey() and getFocusKey() could sometimes return a ContentBlock instead of the key string.

To test

1. Select a note, and make sure it is not focused. (Click somewhere outside the editor)
2. In a different client, edit that same note.

→ Should not result in a WSOD

[roundhill]

Looks good and tested well for me.