Causes of GWT failing

A setter/getter is not defined for some attribute in some class of the model.

Remember, for each attribute annotated with @Param, it must exist a getter and a setter for the class, with the same name as the field. For example, for:

@Param
private boolean enabled;

There must be:

public boolean isEnabled(){
     return enabled;
}
// getEnabled() would also be valid
public void setEnabled(boolean enabled){
     this.enabled = enabled;
}

An enum is missing the @Reflectable

In order to instantiate an Enum in GWT, all enum must been annotated with @Reflectable

@Reflectable
public enum Comparator {
	GREATER, GREATER_EQUAL, EQUAL, LESS_EQUAL, LESS, DIFFERENT
}