Fix qt warning

Try to call deleteLater on a nullptr would cause qt print warning message: "QCoreApplication::notify: Unexpected null receiver"

src/motionclipwidget.cpp

@@ -47,6 +47,7 @@ void MotionClipWidget::setClip(SkeletonMotionClip clip)
 
 void MotionClipWidget::reload()
 {
+    if (nullptr != m_reloadToWidget)
         m_reloadToWidget->deleteLater();
     m_reloadToWidget = new QWidget(this);
     m_reloadToWidget->setContentsMargins(1, 0, 0, 0);