Fix palettes-related crashes

- Fix a crash on showing a tooltip after drag-and-drop in some cases
 - Fix a crash on program exit on Windows

mscore/musescore.cpp

@@ -1960,6 +1960,12 @@ MuseScore::~MuseScore()
             autoUpdater->cleanup();
 
       delete synti;
+
+      // A crash is possible if paletteWorkspace gets
+      // deleted before paletteWidget, so force the widget
+      // be deleted before paletteWorkspace.
+      delete paletteWidget;
+      paletteWidget = nullptr;
       }
 
 //---------------------------------------------------------

mscore/qml/nativetooltip.h

@@ -33,7 +33,7 @@ class QmlNativeToolTip : public QObject {
       Q_PROPERTY(QString text READ text WRITE setText)
 
       QWidget* _widget;
-      QQuickItem* _item = nullptr;
+      QPointer<QQuickItem> _item = nullptr;
       QString _text;
       QString _lastShownText;
       QTimer _timer;