Fix crash when quitting with floating levelstrip

toonz/sources/toonz/filmstrip.cpp

@@ -486,25 +486,27 @@ void FilmstripFrames::hideEvent(QHideEvent *) {
   // active viewer change
   disconnect(app, SIGNAL(activeViewerChanged()), this, SLOT(getViewer()));
 
-  // if the level strip is floating during shutting down Tahoma2D
-  // it can cause a crash disconnecting from the viewer which was already
-  // destroyed.
-  if (m_viewer && m_viewer->isValid()) {
+  disconnectViewer();
+}
+
+//-----------------------------------------------------------------------------
+
+void FilmstripFrames::disconnectViewer() {
+  if (!m_viewer) return;
+
   disconnect(m_viewer, SIGNAL(onZoomChanged()), this, SLOT(update()));
   disconnect(m_viewer, SIGNAL(refreshNavi()), this, SLOT(update()));
+  disconnect(m_viewer, SIGNAL(viewerDestructing()), this,
+             SLOT(disconnectViewer()));
   m_viewer = nullptr;
 }
-}
 
 //-----------------------------------------------------------------------------
 
 void FilmstripFrames::getViewer() {
   bool viewerChanged = false;
   if (m_viewer != TApp::instance()->getActiveViewer()) {
-    if (m_viewer) {
-      disconnect(m_viewer, SIGNAL(onZoomChanged()), this, SLOT(update()));
-      disconnect(m_viewer, SIGNAL(refreshNavi()), this, SLOT(update()));
-    }
+    disconnectViewer();
     viewerChanged = true;
   }
 
@@ -513,6 +515,8 @@ void FilmstripFrames::getViewer() {
   if (m_viewer && viewerChanged) {
     connect(m_viewer, SIGNAL(onZoomChanged()), this, SLOT(update()));
     connect(m_viewer, SIGNAL(refreshNavi()), this, SLOT(update()));
+    connect(m_viewer, SIGNAL(viewerDestructing()), this,
+            SLOT(disconnectViewer()));
     update();
   }
 }

toonz/sources/toonz/filmstrip.h

@@ -172,6 +172,7 @@ protected slots:
   void comboBoxToggled(bool);
   void navigatorToggled(bool);
   void levelSelected(int);
+  void disconnectViewer();
 
 private:
   // QSS Properties

toonz/sources/toonz/sceneviewer.cpp

@@ -852,6 +852,8 @@ SceneViewer::~SceneViewer() {
 
   int ret = l_contexts.erase(m_currentContext);
   if (ret) TGLDisplayListsManager::instance()->releaseContext(m_currentContext);
+
+  emit viewerDestructing();
 }
 
 //-------------------------------------------------------------------------------

toonz/sources/toonz/sceneviewer.h

@@ -498,6 +498,7 @@ signals:
   void refreshNavi();
   // for updating the titlebar
   void previewToggled();
+  void viewerDestructing();
 };
 
 // Functions