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Change EntityBase Type Hint to ContentEntityBase
This changes instances of the EntityBase typehinting of the addExistingRow method in the ComposeWidgetInterface.php and EntityComposeWidget.php files to ContentEntityBase. I believe that this was probably the intent of this typehint. ContentEntityBase should still support all of the entity types I believe we intend to support with this widget.
Typehinting EntityBase results in an error since the Assembly class, at least in the testing case I used, was being passed.