GWT com.google.gwt.core.client.JavaScriptException: (null): null

I recently discovered this problem when doing GWT 2.0 tests in develoment mode. (Other reports of the problem.)
I've seen it arise in calls to constructors to GWT UiObjects/Widgets, such as FlexTables, when I restrict GWT to using only a particular browser, with a line in the module definition such as

<set-property name="user.agent" value="ie6">

Just remove that line, because the headless browser the htmltest framework uses is incompatible with IE6.

Glenn has more details.