It seems like you are loosely comparing $this->primaryKey of type null|string against null; this is ambiguous if the string can be empty. Consider using a strict comparison === instead.
It seems like hasManager() must be provided by classes using this trait. How about adding it as abstract method to this trait?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation