fix: getScreenRect is empty

QtScrcpy/device/ui/videoform.cpp

@@ -329,14 +329,16 @@ QRect VideoForm::getScreenRect()
     if (!win) {
         return screenRect;
     }
+
     QWindow *winHandle = win->windowHandle();
-    if (!winHandle) {
+    QScreen *screen = QGuiApplication::primaryScreen();
-        return screenRect;
+    if (winHandle) {
+        screen = winHandle->screen();
     }
-    QScreen *screen = winHandle->screen();
     if (!screen) {
         return screenRect;
     }
+
     screenRect = screen->availableGeometry();
     return screenRect;
 }