It is a similar issue as this:
Try that gmic_gimp_qt_mint19 version which is basically ubuntu 18.04
Since I do not want to screw up my (k)ubuntu 18.04 installation which is a Gimp 2.8 + 2.10 flatpack, In a ubuntu 18.10 VM that plugin looks like this. https://i.imgur.com/UB5ggf9.jpg
I did have to install libqt5widgets packages, which you might/might not already have. https://i.imgur.com/Lmu28z5.jpg
Anything not working? Always try a running a gimp --verbose in a terminal and look at the output.