It seems like $name can also be of type null; however, parameter $name of Chinstrap\Core\Contracts\Sources\Source::find() does only seem to accept string, maybe add an additional type check?
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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if (!$document = $this->source->find(/** @scrutinizer ignore-type */ $name)) {