It seems like $taxon can also be of type null; however, parameter $taxon of BitBag\SyliusCmsPlugin\E...reInterface::addTaxon() does only seem to accept Sylius\Component\Core\Model\TaxonInterface, 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