MaskGraphicsScene: fix switching between data in mask mode

GUI/View/Mask/MaskGraphicsScene.cpp

@@ -183,9 +183,8 @@ void MaskGraphicsScene::cancelCurrentDrawing()
 
 void MaskGraphicsScene::resetScene()
 {
-    ASSERT(m_selectionModel);
     m_block_selection = true;
-    m_selectionModel->clearSelection();
+
     clearSelection();
 
     clear();
@@ -215,6 +214,9 @@ void MaskGraphicsScene::onSessionSelectionChanged(const QItemSelection&, const Q
     if (m_block_selection)
         return;
 
+    if (!m_selectionModel)
+        return;
+
     m_block_selection = true;
 
     for (auto it = m_ItemToView.begin(); it != m_ItemToView.end(); ++it) {
@@ -232,6 +234,9 @@ void MaskGraphicsScene::onSceneSelectionChanged()
     if (m_block_selection)
         return;
 
+    if (!m_selectionModel)
+        return;
+
     m_block_selection = true;
 
     m_selectionModel->clearSelection();