Delinking sometimes has "undefined" in the edit summary

[NovemLinguae]

Reported by Steel1943 at https://en.wikipedia.org/wiki/Wikipedia_talk:XFDcloser#XFDcloser_creating_erroneous_edit_summaries_and_linking_to_Undefined_after_AfD%2FPROD_closures

Steps to reproduce (needs confirming)

• go to a page that doesn't exist
• make sure the page is linked from at least one other page
• More -> XFDC Unlink

What happens?

• edit summary of the edit removing the link has "Undefined" in it

What should happen instead?

• edit summary of the edit removing the link should be normal

Notes

• Diff: https://en.wikipedia.org/w/index.php?title=H._Jane_Brockmann&diff=prev&oldid=1247251165

[NovemLinguae]

Can XFDcloser even close prods? Or does it have a standalone backlink removal tool? Per Steel1943's question on the talk page, what would explain the prod deletion being done in Twinkle, and the delinking being done in XFDcloser, and having the same timestamp (at least down to the minute, not sure if it's down to the second)?

Prod deletion: https://en.wikipedia.org/w/index.php?title=Special:Log&logid=164647756

Backlink removal: https://en.wikipedia.org/w/index.php?title=H._Jane_Brockmann&diff=prev&oldid=1247251165