I got a Javax.swing.plaf.synth.SynthComboBoxUI$SynthComboBoxRenderer Exception

You're probably running XStudio from a Linux computer on a distribution where Java look'n feel is missing an implementation for this class. You can just force using the cross-platform look'n feel to workaround the issue. In this case, you can enforceXStudio to use the cross-platform look and feel. On the fat client, add the following option to the command line:

--overrideLf javax.swing.plaf.metal.MetalLookAndFeel

If you're launching XStudio from a browser, you'll need to set this server-side in the xstudio.jnlp file.
<argument>--overrideLf</argument> <argument>javax.swing.plaf.metal.MetalLookAndFeel</argument> More details are available here.