GWT does some really neat things.
But it also does some really frustrating things. Such as hiding compilation errors behind inscrutable messages about failing to instantiate a class via deferred binding.
For example, if you create a UiBinder class that uses another class that uses another class and so on, and down the line somewhere you've used a Java feature that GWT does not support, such as the implicit <> generics notation for instantiation of a generic typed variable, it will fail at the top level UiBinder class without telling you what failed.
Furthermore, if you try to use such a UiBinder class in another UiBinder class, it will fail to compile that second class claiming that the first cannot even be resolved as a type.
To figure out what the heck is going on, you have to crank the logging level of GWT's compiler way up and sift through pages and pages of spam to find the actual error you're looking for.