It seems like $casProtocol->handleProxyCallback() can also be of type null; however, parameter $psrResponse of Symfony\Bridge\PsrHttpMe...rface::createResponse() does only seem to accept Psr\Http\Message\ResponseInterface, 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