fixed an issue where the application would crash when trying to drag and drop any unexpected object on the thumbnails view

src/gui/UBDocumentNavigator.cpp

@@ -532,6 +532,8 @@ void UBDocumentNavigator::dragMoveEvent(QDragMoveEvent *event)
                            item->pos().y() + item->boundingRect().height() * scale / 2);
 
         bool dropAbove = mapToScene(position.toPoint()).y() < itemCenter.y();
+        if (mDropSource)
+        {
             bool movingUp = mDropSource->sceneIndex() > item->sceneIndex();
             qreal y = 0;
 
@@ -554,7 +556,7 @@ void UBDocumentNavigator::dragMoveEvent(QDragMoveEvent *event)
                 }
             }
         }
-
+    }
     event->acceptProposedAction();
 }
 
@@ -562,8 +564,11 @@ void UBDocumentNavigator::dropEvent(QDropEvent *event)
 {
     Q_UNUSED(event);
 
+    if (mDropSource && mDropTarget)
+    {
         if (mDropSource->sceneIndex() != mDropTarget->sceneIndex())
             UBApplication::boardController->moveSceneToIndex(mDropSource->sceneIndex(), mDropTarget->sceneIndex());
+    }
 
     mDropSource = NULL;
     mDropTarget = NULL;