Windows: Fix crash when focus window does not have a native window.

Check and warn in that case.

Change-Id: Ic513334b5aa48e1c7e44685c30da3e9be52c3c52
Task-number: QTBUG-43833
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/plugins/platforms/windows/qwindowsinputcontext.cpp

@@ -209,7 +209,7 @@ void QWindowsInputContext::setFocusObject(QObject *object)
         imeNotifyCancelComposition(m_compositionContext.hwnd);
 
     const QWindow *window = QGuiApplication::focusWindow();
-    if (object && window) {
+    if (object && window && window->handle()) {
         QWindowsWindow *platformWindow = QWindowsWindow::baseWindowOf(window);
         if (inputMethodAccepted()) {
             // Re-enable IME by associating default context saved on first disabling.