fix: crashed on palette destroyed

do not set value if control's window is null
Issue: linuxdeepin/dtk#193

src/private/dquickcontrolpalette.cpp

@@ -937,7 +937,7 @@ void DQuickControlColorSelector::notifyColorPropertyChanged()
 
 void DQuickControlColorSelector::updatePropertyFromName(const QByteArray &name, const DQuickControlPalette *palette)
 {
-    if (QCoreApplication::closingDown())
+    if (QCoreApplication::closingDown() || (m_control && !m_control->window()))
         return;
     Q_ASSERT(!name.isEmpty());
     QColor color;