The was only one change to gr-wxgui between 3.6.4.1 and 3.6.4.2, which was a bug fix for operation on OSX. I've pushed a branch 'test-wxgui-fix' on my github repo:
https://github.com/jmcorgan/gnuradio.git...that reverts this to see if that bug fix was the cause of the Kubuntu issue you are seeing.
The branch, however, is off version 3.6.5. If you try this and it corrects the issue you're seeing, then we'll have to figure out a way to address the original OSX issue without creating this new one. Whatever that is, it would eventually make it into a 3.6.5.1 release.