Concurrent editing after deleting content causes endless error 500

Description

Scenario #1:
1. Create a new page.
2. User1 & User2 open the page for editing in 2 browsers.
3. User1 deletes it permanently.
4. User2 edits its property.

Scenario #2:
1. Publish a page then create a new draft version.
2. User1 & User2 open the draft version for editing in 2 browsers.
3. User1 deletes the draft version (or reverts to published).
4. User2 edits its property.

Expected:
Show a 404 script error once. There should be also an error/warning message in the UI. After that, publishing is disabled.

Actual:
Shows a script error 500 for every minute. This error message appears in the UI "Offline, can't save".

Observed: If User2 publishes the page at step #4, it shows the 404 error once with a friendly message "Content with id XXX_XXX was not found". This is the correct behavior.