It seems like $this->fileDataFactory->create($object) can also be of type null; however, parameter $fileData of Silverback\ApiComponentB...nterface::setFileData() does only seem to accept Silverback\ApiComponentBundle\Dto\File\FileData, 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