Can you explain the exact steps to reproduce this error message? It would be great if I could reproduce it myself. That would be the first step to fix it.
And what version of Glyphs do you use? Can you try the latest cutting edge version?
Also, if using Glyphs without plugins and it worked (better), it would be great to find out which plugin interferes. So that it can be fixed properly. If you have the time to try to find out, that would be much appreciated.