WSOD: The subform is not contained by the given parent form

@Mav_fly
We need more details to reproduce like the readonly HTML field has been added to which entity type form?

I added the field to a profile "customer" and than I added the readonly field to an article. And in both cases I got the WSOD.

@Mav_fly thank you for details, now I can reproduce it.
Possible related issues: Using $form_state->getValue() in BlockBase's blockForm throws "subform and parent form must contain the #parents property" exception →

Issue status has been changed to Needs work.

The problem here is that Readonly Html field type settings form build method ReadonlyHtmlFieldItem::fieldSettingsForm returns the complete form and this behavior has been changed recently in core parent method that returns the subform instead, so core is expecting a subform from ReadonlyHtmlFieldItem::fieldSettingsForm method, to fix this, first we retrieve the subform from the parent and then we use the retrieved subform to build Readonly Html field type settings form.
Please try out the attached patch and confirm if it resolves the issue. Thanks!

I applied the patch, and add a Readonly Html field without getting a WSOD.
So I guess to problem is solved!!

@b.khouy thanks for the fast reaction!!

@Mav_fly Thank you for testing and confirming that the patch resolved your issue!
The patch has been committed and the related merge request has been successfully merged, a new release, version 1.0.5, has been created including this fix.