The expression return $this->repository...y($this->user->getId()) could return the type Yiisoft\Auth\IdentityInterface which includes types incompatible with the type-hinted return App\Entity\User|null. Consider adding an additional type-check to rule them out.
It seems like $this->user->getId() can also be of type null; however, parameter $id of App\Repository\UserRepository::findIdentity() does only seem to accept string, maybe add an additional type check?
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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-type annotation