As many of you have pointed out in countless emails and user group posts: Currently mgwt and the GWT Designer don`t really get along. There are some workarounds to make it work, but they are pretty ugly.
The cause behind this is that GWT Designer was not able to run property providers from GWT. But mgwt heavily relies on them to detect your platform and many other important things.
We discussed what changes would be necessary to mgwt to make it work and quickly saw that it can not be done in a pleasant way. Instead I decided to bring it up in the gwt contributer list and see if someone from google could instead make GWT Designer work with custom properties.
There already was an open issue and @kschlegov fixed it today. Many thanks to him for such a quick fix and I hope you will be enjoying a working GWT Designer starting with the next release of GWT Designer.