I just encountered the same issue, my asset1 references to asset2 that never exists anymore, and the reference viewer says asset2 is None.
And such approach fix it->Create a dummy file asset2 at where it would exist->Right click on the dummy asset2 and check the reference viewer you will find the asset1 references to this dummy asset2->Open asset1, and click save->Restart the editor, have a look at the reference viewer of asset2 you will find nobody references it anymore and you can delete dummy asset2 safely now
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