Core/AreaTrigger: Fix crashes happening if areatrigger is despawned in OnUnitEnter script

Port From (https://github.com/TrinityCore/TrinityCore/commit/9827685634c8487936b8e59cdece596c5abbb145)

Source/Game/Entities/AreaTrigger/AreaTrigger.cs

@@ -858,6 +858,15 @@ namespace Game.Entities
             DoActions(unit);
 
             _ai.OnUnitEnter(unit);
+
+            // OnUnitEnter script can despawn this areatrigger
+            if (!IsInWorld)
+                return;
+
+            // Register areatrigger in Unit after actions/scripts to allow them to determine
+            // if the unit is in one or more areatriggers with the same id
+            // without forcing every script to have additional logic excluding this areatrigger
+
             unit.EnterAreaTrigger(this);
         }