gwtmockito with requestbuilder

Question

This is a part of my class, I want to test:

public class PrefPanel extends Composite {

  private static PrefPanelUiBinder uiBinder =  GWT.create(PrefPanelUiBinder.class);

  interface PrefPanelUiBinder extends UiBinder<Widget, PrefPanel> {}

  public PrefPanel(GlobalParams globalParams) {
    initWidget(uiBinder.createAndBindUi(this));

    String url = URL.encode(globalParams.getBaseUrl() + "book.html");
    RequestBuilder builder = new RequestBuilder(RequestBuilder.POST, url);
    try {
      Request response = builder.sendRequest(jsonString, new RequestCallback() {

      @Override
      public void onError(Request request, Throwable exception) {
        displayError("Error");
      }

      @Override
      public void onResponseReceived(Request request, Response response) {
        updateBookList(response.getText());
      }
    });
  } catch (RequestException e) {
    displayError("Error");
  }

Here is a part of my test class:

@RunWith(GwtMockitoTestRunner.class)
public class PositionServiceTest {

@Mock RequestBuilder builder;
@Mock GlobalParams globalParams;
@Mock URL url;
private PrefPanel prefPanel;

@Before
public void setup() {
  GwtMockito.useProviderForType(RequestBuilder.class, new FakeProvider() {
    @Override
    public Object getFake(Class aclass) {
      return builder;
    }
  });

when(globalParams.getBaseUrl()).thenReturn("http://localhost/");
prefPanel = new PrefPanel(globalParams);
...

When I start to debug I get an error message: - url cannot be empty - java.lang.IllegalArgumentException - at com.google.gwt.http.client.StringValidator.throwlfEmptyOrNull(StringValidator.java:52) - ...

The error occurs on the line where I create the RequestBuilder (new RequestBuilder). I have no idea how to create a new instance of RequestBuilder. Could you give me a clue?

I have heard that gwtmockit can't handle constructors. Is there a way to avoid the new RequestBuilder? Do I have to use powermockito?