I had searched for a word in all files and collected the matches in a Find All pane. Then I was clicking on them and removing them using the context menu.
This seems to happen if no match is selected, and then the user directly
right-clicks on a match.
If the user first selects a match with a left click and then right-clicks,
it works.
Still does not allow right-click selection+popup menu, at least on Linux. Works on the Mac.
This may be Linux-only.
See https://sourceforge.net/tracker/index.php?func=detail&aid=3040733&group_id=44253&atid=438935