Simple validation crashing on just one property (+other validation issues)

I’ve switched to sub view models that accept null and bound them to my sections. I get warnings of possible null value in markup in many places. I suspect this is because nullable context is enabled at the project level (it was convenient to leave it like that).
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Do you have any suggestions for getting rid of this?
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PS:
Disabling it at the top of the .cs view model file of the MarkupControl does not resolve this.
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