fix: #3596331 Fix TypeError in getWidgetState() when #field_parents is missing...

Steffen Rühlmannrequested to merge
issue/paragraphs_sets-3596331:3596331-php-exception into 3.0.x

fix: #3596331 Fix TypeError in getWidgetState() when #field_parents is missing on pre-built widget element

Closes #3596331

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