sync: from linuxdeepin/dtkwidget

Synchronize source files from linuxdeepin/dtkwidget.

Source-pull-request: linuxdeepin/dtkwidget#632

Author: deepin-ci-robot

src/widgets/dtitlebar.cpp

@@ -164,7 +164,7 @@ class DTitlebarPrivate : public DTK_CORE_NAMESPACE::DObjectPrivate
     QAction             *darkThemeAction  = nullptr;
 #endif
 
-    QWindow            *targetWindowHandle = Q_NULLPTR;
+    QPointer<QWindow>            targetWindowHandle;
 
     Qt::WindowFlags     disableFlags;
     bool                mousePressed    = false;