Fix type error when merging blocks #4791
Merged
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What this is
Fixes #4580. This was a regression introduced by #4395.
When we merge two blocks by pressing the BACKSPACE KEY, the TinyMCE instance belonging to the second block is destroyed. This results in a type error raised by a TinyMCE event handler that is listening to the same event.
By immediately preventing propagation, we can stop TinyMCE's handler from ever running.
How to test
Check that the bug is fixed:
Check that we didn't break #4395: