Unit Testing Views in WPF

I have used Mock to unit test ViewModels. My problem is with the views. I have to test the KeyUp event of a textbox.

In my ViewTest class, I have the following code.

View1 ViewTarget = null;

[TestInitialize()]
public void MyTestInitialize()
{
  ViewTarget = new Veiw1();
}

[TestMethod]
    public void searchTextBox_KeyUpTest()
    {

        TextBox element = new TextBox();
        element.RaiseEvent(
                 new KeyEventArgs(
                  Keyboard.PrimaryDevice,
                  Keyboard.PrimaryDevice.ActiveSource,
                  0,
                  Key.Enter) { RoutedEvent = Keyboard.KeyUpEvent }
                       );
    }

What do I pass as the input source? Keyboard.PrimaryDevice.ActiveSource returns null. I tried mocking the PresentationSource for input source but throws a ProxyGenerationException.