Problem with Translation of own Entities after Update

i do have a plugin with own Entity-Elements.
at this entity i have a TranslationsAssociationField

in 6.4.8 new/update an entry works fine.

after update to 6.4.20 i get an error „Call to a member function is() on null“

in trace it references to class: „Shopware\Core\Framework\DataAbstractionLayer\Write\Command\WriteCommandQueue“

are there any changes?
how can i find the exact problem in the complete error which is returned?