Changing configured element in single block list causes UX issues.

[Migaroez]

Which Umbraco version are you using? (Please write the exact version, example: 10.1.0)

15/dev (15.3.0-rc.preview.3)

Bug summary

There is some unwanted and unstable behavior when swapping elements in a single block list that already has content

Specifics

Related to #16428

A few issues seem to coexist that make the editing experience for single item block lists unusable in the scenario where to element type for the single block is reconfigured.

single.block.15.mp4

Steps to reproduce

Setup 2 simple element types, in the example one with a single RTE field and one with a single textstring field
Setup a blocklist with the title block assigned to it as the single block and configure the blocklist for single block usage with min max set to 1
Follow the video supplied in the specifics, but it comes down to

• fill in the content and save
• swap the blocklist block to the other element type
• try to update the content (first time crashes)
• try to publish the content
• swap back the doctype
• check the content, it will have its originally saved value still active

Expected result / actual result

I expect to have a choice to remove incompatible data from the block list as in before v14
I expect the UI not to crash
I expect to be able to update the date in the block list trough the UI so it can pass publishing validation
I expect no errors in the dev console