I think it's definitely possible to get a nsDisplaySubdocument for which it's mSubDoc frames returns null from GetSubdocumentPresShellForPainting (and hence get a null presshell in IncrementSubDocPresShellPaintCount). In nsSubDocumentFrame::BuildDisplayList we build a nsDisplaySubdocument that doesn't want to be flattened for all root content documents, even if the subdoc doesn't have a root frame (which happens briefly early in the creation of the layout structures for a document). GetSubdocumentPresShellForPainting could definitely return null if the subdoc doesn't have a root frame.
(We can get a presshell via a view now, which was not true when GetSubdocumentPresShellForPainting was written. So we could change GetSubdocumentPresShellForPainting to get a presshell in the case of no subdoc root frame, but that means changing the main painting path too, and I think I would like to avoid that: painting a subdoc without a root frame is useless anyways and the change might cause other problems.)
So I think just null checking this for now is the right move, unless/until we get more evidence that there is an actual bug here other than a missing null check at this site.