Adding a "helper assert" for when you can create QWindow

maybe if your trying to instansiate a QWindow before the QApplication has
been instansiated.

Change-Id: I68ed4ad1dbdfedc242a7cb27c1381d50a3d3dfa4
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/kernel/qwindow.cpp

@@ -64,6 +64,11 @@ QWindow::QWindow(QScreen *targetScreen)
     d->screen = targetScreen;
     if (!d->screen)
         d->screen = QGuiApplication::primaryScreen();
+
+    //if your applications aborts here, then chances are your creating a QWindow before the
+    //screen list is populated.
+    Q_ASSERT(d->screen);
+
     QGuiApplicationPrivate::window_list.prepend(this);
 }