Fixed crash when deleting multiple objects with manual drawing order

Due to the way the change was handled in different places, the items
could get updated after they were deleted, leading to dereferencing a
null pointer.

Fixed by deleting items individually.

Closes mapeditor#2844

src/tiled/mapitem.cpp

@@ -368,9 +368,11 @@ void MapItem::documentChanged(const ChangeEvent &change)
     case ChangeEvent::LayerChanged:
         layerChanged(static_cast<const LayerChangeEvent&>(change).layer);
         break;
-    case ChangeEvent::MapObjectsAboutToBeRemoved:
-        deleteObjectItems(static_cast<const MapObjectsEvent&>(change).mapObjects);
+    case ChangeEvent::MapObjectAboutToBeRemoved: {
+        auto &e = static_cast<const MapObjectEvent&>(change);
+        deleteObjectItem(e.objectGroup->objectAt(e.index));
         break;
+    }
     case ChangeEvent::MapObjectsChanged:
         syncObjectItems(static_cast<const MapObjectsChangeEvent&>(change).mapObjects);
         break;
@@ -563,15 +565,11 @@ void MapItem::objectsInserted(ObjectGroup *objectGroup, int first, int last)
 /**
  * Removes the map object items related to the given objects.
  */
-void MapItem::deleteObjectItems(const QList<MapObject*> &objects)
+void MapItem::deleteObjectItem(MapObject *object)
 {
-    for (MapObject *o : objects) {
-        auto i = mObjectItems.find(o);
-        Q_ASSERT(i != mObjectItems.end());
-
-        delete i.value();
-        mObjectItems.erase(i);
-    }
+    auto item = mObjectItems.take(object);
+    Q_ASSERT(item);
+    delete item;
 }
 
 /**

src/tiled/mapitem.h

@@ -107,7 +107,7 @@ class MapItem : public QGraphicsObject
     void tilesetReplaced(int index, Tileset *tileset);
 
     void objectsInserted(ObjectGroup *objectGroup, int first, int last);
-    void deleteObjectItems(const QList<MapObject*> &objects);
+    void deleteObjectItem(MapObject *object);
     void syncObjectItems(const QList<MapObject*> &objects);
     void objectsIndexChanged(ObjectGroup *objectGroup, int first, int last);