Avoid crash when accessing the main windget position

src/blackgui/dockwidget.cpp

@@ -659,7 +659,7 @@ namespace BlackGui
         QPoint pos = CGuiUtility::mainWidgetGlobalPosition();
         if (pos.isNull())
         {
-            pos = sGui->currentScreen()->geometry().center() - this->rect().center();
+            pos = CGuiApplication::currentScreen()->geometry().center() - this->rect().center();
         }
 
         const int x = pos.x() + m_offsetWhenFloating.x();

src/blackgui/guiapplication.cpp

@@ -355,7 +355,8 @@ namespace BlackGui
     QRect CGuiApplication::currentScreenGeometry()
     {
         const QScreen *s = currentScreen();
-        return s->geometry();
+        if (s) return s->geometry();
+        return {};
     }
 
     void CGuiApplication::modalWindowToFront()

src/blackgui/guiutility.cpp

@@ -317,9 +317,13 @@ namespace BlackGui
 
     QPoint CGuiUtility::mainWidgetGlobalPosition()
     {
-        CEnableForFramelessWindow *mw = CGuiUtility::mainFramelessEnabledWindow();
+        QWidget *mw = CGuiUtility::mainApplicationWidget();
-        if (!mw || !mw->getWidget()) { QPoint(); }
+        if (mw) { return mw->pos(); }
-        return mw->getWidget()->pos(); // is main window, so not mapToGlobal
+
+        // fallback, can be mfw it is not found
+        CEnableForFramelessWindow *mfw = CGuiUtility::mainFramelessEnabledWindow();
+        if (!mfw || !mfw->getWidget()) { return QPoint(); }
+        return mfw->getWidget()->pos(); // is main window, so not mapToGlobal
     }
 
     QString CGuiUtility::replaceTabCountValue(const QString &oldName, int count)
@@ -353,7 +357,7 @@ namespace BlackGui
                     delete widget;
                 }
             }
-            else {delete item;}
+            else { delete item; }
         }
 
         // then finally