Hi!
gMKVExtractGUI 2.4.0.0
mkvtoolnix 28.2.0 (x64)
Win 7 x64 Ultimate
If one item is selected, there is no problem.
https://i.imgur.com/uFztT6V.png
If two or more items are selected, then an error.
https://i.imgur.com/forsNZd.png
https://i.imgur.com/BXDaUU6.png