After installing the shiny new version of Eclipse entitled 'Luna' on a Kubuntu 14.10 box, I found that it crashed/hung when trying to import/clone a Git repository. Starting the application from the command-line triggered the following notice before the crash:
java: /build/buildd/gtk2-engines-oxygen-1.4.5/src/animations/oxygencomboboxdata.cpp:87: void Oxygen::ComboBoxData::setButton(GtkWidget*): Assertion `!_button._widget' failed.
So, it appeared to be a theme-related issue. Googling around led me to this error report which confirmed that it was a GTK (widget) issue. The quick suggested workaround is to simply not use the Oxygen theme mentioned in the error message. This can be done by going into System Settings, Application Appearance, choosing GTK on the left, and changing the GTK2 Theme currently being used from Oxygen to something else, and clicking Apply. In my case, the only available option was Raleigh (which turned out to be rather ... palaeolithic in style).
Changing the GTK2 theme fixed the Eclipse issue for me! If you don't like the Raleigh theme, you can use the Get New Themes button on the same settings page to download prettier themes. You can also install ones that are available in the Ubuntu repository. I tried Aurora (in the package
gtk2-engines-aurora) which looked pretty decent.
All that said, while we wait for the powers-that-be to update the packages appropriately, there is another fix for this: Open a terminal and edit the file
/usr/share/themes/oxygen-gtk/gtk-2.0/gtkrc and toggle the value for
GtkComboBox::appears-as-list from 1 to 0. This fixed the issue for me and is the workaround I'm currently using.
Hope this helps!