If you are a Java Developer the chances you are running Eclipse too is big (I think :S). if it’s the case and you are developing GWT application then the chances you’ve already installed Google Plugin for Eclipse is even bigger, but for some reason Google plugin doesn’t play really well with Eclipse running in Ubuntu machine.
Whenever I install a newer version of Ubuntu the first thing I test after installing OpenJDK and Eclipse if GWT Designer will work this time, and as expected when you install the latest Ubuntu release (it’s 12.04) at the time I wrote this post, GWT Designer won’t work. But luckily there’s a work around.
If you have Eclipse already installed and both Google Plugin and GWT Designer installed, then you are ready to go ;).
This workaround have been tested under the 64bit version of Ubuntu.

1. Download and Install libhunspell-1.2-0 from here.
The package can be installed by running the following command: sudo dpkg -i libhunspell-1.2-0_1.2.14-4_amd64.deb

2. Download and install xulrunner-1.9.2 from here.
The package can be installed by running the following command : sudo dpkg -i sudo dpkg -i xulrunner-1.9.2_1.9.2.28+build1+nobinonly-0ubuntu0.11.04.1_amd64.deb.
3. Open Eclipse and look under Window->Preferences->Window Builder->GWT make sure “Use WebKit for rendering GWT UI (if available)” is unchecked.

4. Create a new GWT web application, then create a UIBinder component to test if GWT Designer is working or not. If you see a black window when you open the designer as shown below:

then there’s one more step needs to be done.
Go to Window->Preferences->WindowBuilder then enable the following option “Disable Preview Window flickering workarounds (Linux only)” option.