Unable to edit the formatter settings (Warning: Trying to access array offset on null in Drupal\field_ui\Form\EntityDisplayFormBase->copyFormValuesToEntity())

@grevil, I think best will be to step-debug that...
Hope it wasn't caused by #3520976: Fix autoPlay vs. autoplay typo in code but the module wasn't installed there before, so typically *should* not be the case.

I got this message now when trying to configure the formatter:
Warning: Undefined array key "field_mime_type" in Drupal\field_ui\Form\EntityDisplayFormBase->copyFormValuesToEntity() (Zeile 615 ...

@grevil: I tried to unblock @thomas.frobieter and commented out all the field formatter settings code, but with no luck... the issue persists. So I'm wondering if we're inheriting this issue from FileMediaFormatterBase maybe?

Hopefully you'll be able to step-debug it easily, this is really strange... I thought the field selection for the poster image was maybe trying to handle non-existing fields or base-fields... but commenting out the code should have fixed that...

Just found these possible similar issues:

#3166929: Ajax error: Trying to access array offset on value of type null
#3238558: Multiple 'undefined index' and ' Trying to access array offset' notices in log when clicking on cog wheel
#3231869: Form alter removes '#submit' and breaks Drupals expected array structure

Fixed through #3521021: Add parent calls, adjust templates and fix formatter settings