Fixed a crash in document mode

src/domain/UBGraphicsItemUndoCommand.cpp

@@ -159,7 +159,7 @@ void UBGraphicsItemUndoCommand::undo()
 
 }
 
-void UBGraphicsItemUndoCommand::redo()
+/*void UBGraphicsItemUndoCommand::redo()
 {
     // the Undo framework calls a redo while appending the undo command.
     // as we have already plotted the elements, we do not want to do it twice
@@ -238,3 +238,4 @@ void UBGraphicsItemUndoCommand::redo()
         mFirstRedo = false;
     }
 }
+*/

src/domain/UBGraphicsItemUndoCommand.h

@@ -52,7 +52,7 @@ class UBGraphicsItemUndoCommand : public UBAbstractUndoCommand
 
     protected:
         virtual void undo();
-        virtual void redo();
+        //virtual void redo();
 
     private:
         UBGraphicsScene* mScene;

src/gui/UBDocumentNavigator.cpp

@@ -142,7 +142,8 @@ void UBDocumentNavigator::onScrollToSelectedPage(int index)
         }
         c++;
     }
-    centerOn(mSelectedThumbnail);
+    if(NULL != mSelectedThumbnail)
+        centerOn(mSelectedThumbnail);
 }
 
 /**