prevent a crash when clearBackground then undo then redo then clearBackground

src/domain/UBGraphicsItemUndoCommand.cpp

@@ -246,6 +246,10 @@ void UBGraphicsItemUndoCommand::redo()
 
                 polygonItem->strokesGroup()->removeFromGroup(polygonItem);
             }
+
+            if (itemLayerType::BackgroundItem == item->data(UBGraphicsItemData::itemLayerType))
+                mScene->setAsBackgroundObject(nullptr);
+            else
                 mScene->removeItem(item);
 
             if (bApplyTransform)

src/domain/UBGraphicsScene.cpp

@@ -1476,6 +1476,7 @@ void UBGraphicsScene::clearContent(clearCase pCase)
         if(mBackgroundObject){
             removeItem(mBackgroundObject);
             removedItems << mBackgroundObject;
+            mBackgroundObject = nullptr;
         }
         break;