Removing a Classic block in the code editor causes Blocks to be "Deleted or unavailable" in the Visual editor

[1naveengiri]

Issue Overview

I switched from visual editor to the code editor, it shows me a message on the already added block
Block has been deleted or is unavailable
although on page refresh the blocks are there and also on post update they also visible in the frontend.

Steps to Reproduce (for bugs)

1. go to edit post page
2. Add few blocks to post content with Gutenberg layout.
3. Change editor from visual to code from the option given beside setting in RHS.
4. add a new block through code editor.
5. switch back from code editor to visual editor and you will see the message Block has been deleted or is unavailable

Check the video below

Screenshots / Video

https://drive.google.com/file/d/1DW4tOQ6007Q1hGciyXm5jmQxA1Nj7nR0/view

[danielbachhuber]

Thanks for the report, @1naveengiri.

To make your screencast more accessible...

1. Add some blocks to the Gutenberg editor

2. Remove the Classic block

3. "Block is deleted or unavailable" message appears

I don't know the underlying implementation well-enough to comment on a solution, but I'm filing against Merge Proposal: Editor because I think this is a significant-enough problem that it should be addressed sooner rather than later. It seems to be similar to #4498

[danielbachhuber]

Closing in favor of #5754, which is an earlier report