Core/Battlegrounds: Unload all grids before destroying battleground object

Port From (https://github.com/TrinityCore/TrinityCore/commit/3fc3cd455b51d2de4ba5a70b7eff15e23b3da481)

Source/Game/BattleGrounds/BattleGround.cs

@@ -54,7 +54,14 @@ namespace Game.BattleGrounds
             Global.BattlegroundMgr.RemoveBattleground(GetTypeID(), GetInstanceID());
             // unload map
             if (m_Map)
-                m_Map.SetUnload();
+            {
+                m_Map.UnloadAll(); // unload all objects (they may hold a reference to bg in their ZoneScript pointer)
+                m_Map.SetUnload(); // mark for deletion by MapManager
+
+                //unlink to prevent crash, always unlink all pointer reference before destruction
+                m_Map.SetBG(null);
+                m_Map = null;
+            }
 
             // remove from bg free slot queue
             RemoveFromBGFreeSlotQueue();

Source/Game/Maps/Map.cs

@@ -1474,7 +1474,7 @@ namespace Game.Maps
             }
         }
 
-        public virtual void UnloadAll()
+        public void UnloadAll()
         {
             // clear all delayed moves, useless anyway do this moves before map unload.
             creaturesToMove.Clear();