Also had this issue this morning, when everything worked fine yesterday. I didn't think I even initiated updates, and I don't update automatically.

Kubuntu 20.04 (Yeah, I need to update but...snaps...). gitk was 1:2.34.1-1ubuntu1.13, git 2.34.1.

The fix from @ivorybabe worked, even though it is probably a temporary fix. I downgraded gitk as he said, but did not change git-gui.