Fix GUI scaling factor on macOS

macOS seems have an unique way to handle widget size, that is, it doesn't require
application to resize widgets manually, Qt will handle the resize job
automatically.

Closes #8841.

src/gui/utils.cpp

@@ -45,16 +45,21 @@ void Utils::Gui::resize(QWidget *widget, const QSize &newSize)
 
 qreal Utils::Gui::screenScalingFactor(const QWidget *widget)
 {
+    if (!widget)
+        return 1;
+
 #ifdef Q_OS_WIN
     const int screen = qApp->desktop()->screenNumber(widget);
     return (QApplication::screens()[screen]->logicalDotsPerInch() / 96);
+#elif defined(Q_OS_MAC)
+    return 1;
 #else
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0))
     return widget->devicePixelRatioF();
 #else
     return widget->devicePixelRatio();
 #endif
-#endif  // Q_OS_WIN
+#endif
 }
 
 QPixmap Utils::Gui::scaledPixmap(const QString &path, const QWidget *widget, const int height)