QGuiApplication::font(): assert that a GUI app instance exists

Task-number: QTBUG-28306
Change-Id: Ia5b01265e7f88fdd8c2738ef03b14e83829625b0
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -2293,6 +2293,7 @@ void QGuiApplication::setPalette(const QPalette &pal)
 */
 QFont QGuiApplication::font()
 {
+    Q_ASSERT_X(QGuiApplicationPrivate::self, "QGuiApplication::font()", "no QGuiApplication instance");
     QMutexLocker locker(&applicationFontMutex);
     initFontUnlocked();
     return *QGuiApplicationPrivate::app_font;