Editor was crashing if multiple, specific block elements were inside list item in loaded/pasted data

[scofalik]

Suggested merge commit message (convention)

Fix: Editor was crashing if multiple, specific block elements were inside list item in loaded/pasted data. Closes ckeditor/ckeditor5#1572.

[scofalik]

https://github.com/ckeditor/ckeditor5-paragraph/issues/43 This may be an alternative solution to the problem

[scofalik]

Maybe I will elaborate more on it: if we would fix autoparagraphing, maybe we don't need to change lists at all. But OTOH we cannot guarantee that every converter will set modelCursor correctly. So proposed solution is still a little safer than relying on modelCursor.

[Mgsy]

I've spent some time pasting different content, including multiple blocks in a list item, and it looks good to me.

[Reinmar]

The code looks good. It works fine as well. I was not able to crash the editor except by doing something when an image is selected in case of very long content (ckeditor/ckeditor5#1578).