Core/AI: Fix crashes caused by charmed Creatures having null AI for 1 map update tick

Port From (https://github.com/TrinityCore/TrinityCore/commit/8c25abca72772e2962bac7f460a567a943388a05)

Source/Game/AI/CoreAI/ScheduledChangeAI.cs

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2012-2020 CypherCore <http://github.com/CypherCore>
+ * 
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+using Framework.Constants;
+using Game.Entities;
+
+namespace Game.AI
+{
+    class ScheduledChangeAI : CreatureAI
+    {
+        public ScheduledChangeAI(Creature creature) : base(creature) { }
+
+        public override void MoveInLineOfSight(Unit unit) { }
+
+        public override void AttackStart(Unit unit) { }
+
+        public override void JustStartedThreateningMe(Unit unit) { }
+
+        public override void JustEnteredCombat(Unit unit) { }
+
+        public override void UpdateAI(uint diff) { }
+
+        public override void JustAppeared() { }
+
+        public override void EnterEvadeMode(EvadeReason why) { }
+
+        public override void OnCharmed(bool isNew) { }
+    }
+}

Source/Game/Entities/Unit/Unit.cs

@@ -196,7 +196,7 @@ namespace Game.Entities
             // All position info based actions have been executed, reset info
             _positionUpdateInfo.Reset();
 
-            if (GetAI() == null && (!IsPlayer() || (IsCharmed() && GetCharmerGUID().IsCreature())))
+            if (HasScheduledAIChange() && (!IsPlayer() || (IsCharmed() && GetCharmerGUID().IsCreature())))
                 UpdateCharmAI();
 
             RefreshAI();
@@ -1223,11 +1223,11 @@ namespace Game.Entities
             bool charmed = IsCharmed();
 
             if (charmed)
-                PushAI(null);
+                PushAI(GetScheduledChangeAI());
             else
             {
                 RestoreDisabledAI();
-                PushAI(null); //This could actually be PopAI() to get the previous AI but it's required atm to trigger UpdateCharmAI()
+                PushAI(GetScheduledChangeAI()); //This could actually be PopAI() to get the previous AI but it's required atm to trigger UpdateCharmAI()
             }
         }
 
@@ -1235,10 +1235,28 @@ namespace Game.Entities
         {
             // Keep popping the stack until we either reach the bottom or find a valid AI
             while (PopAI())
-                if (GetTopAI() != null)
+                if (GetTopAI() != null && GetTopAI() is not ScheduledChangeAI)
                     return;
         }
 
+        UnitAI GetScheduledChangeAI()
+        {
+            Creature creature = ToCreature();
+            if (creature != null)
+                return new ScheduledChangeAI(creature);
+            else
+                return null;
+        }
+
+        bool HasScheduledAIChange()
+        {
+            UnitAI ai = GetAI();
+            if (ai != null)
+                return ai is ScheduledChangeAI;
+            else
+                return true;
+        }
+        
         public bool IsPossessedByPlayer()
         {
             return HasUnitState(UnitState.Possessed) && GetCharmerGUID().IsPlayer();