Core/AI: Fix crashes caused by charmed Creatures having null AI for 1 map update tick (#27434)

Implement using ScheduledChangeAI instead of nullptr to signal a required AI change
# Conflicts:
#	src/server/game/Entities/Unit/Unit.cpp
#	src/server/game/Entities/Unit/Unit.h

src/server/game/Entities/Unit/Unit.cpp

@@ -65,6 +65,7 @@
 #include "PlayerAI.h"
 #include "QuestDef.h"
 #include "ReputationMgr.h"
+#include "ScheduledChangeAI.h"
 #include "Spell.h"
 #include "SpellAuraEffects.h"
 #include "SpellAuras.h"
@@ -543,7 +544,7 @@ void Unit::Update(uint32 p_time)
         _healthRegenerationTimer += UNIT_HEALTH_REGENERATION_INTERVAL;
     }
 
-    if (!GetAI() && (GetTypeId() != TYPEID_PLAYER || IsCharmed()))
+    if (HasScheduledAIChange() && (GetTypeId() != TYPEID_PLAYER || (IsCharmed() && GetCharmerGUID().IsCreature())))
         UpdateCharmAI();
     RefreshAI();
 }
@@ -10354,11 +10355,11 @@ void Unit::ScheduleAIChange()
     bool const charmed = IsCharmed();
 
     if (charmed)
-        PushAI(nullptr);
+        PushAI(GetScheduledChangeAI());
     else
     {
         RestoreDisabledAI();
-        PushAI(nullptr); //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()
     }
 }
 
@@ -10366,10 +10367,26 @@ void Unit::RestoreDisabledAI()
 {
     // Keep popping the stack until we either reach the bottom or find a valid AI
     while (PopAI())
-        if (GetTopAI())
+        if (GetTopAI() && dynamic_cast<ScheduledChangeAI*>(GetTopAI()) == nullptr)
             return;
 }
 
+UnitAI* Unit::GetScheduledChangeAI()
+{
+    if (Creature* creature = ToCreature())
+        return new ScheduledChangeAI(creature);
+    else
+        return nullptr;
+}
+
+bool Unit::HasScheduledAIChange() const
+{
+    if (UnitAI* ai = GetAI())
+        return dynamic_cast<ScheduledChangeAI*>(ai) != nullptr;
+    else
+        return true;
+}
+
 void Unit::AddToWorld()
 {
     WorldObject::AddToWorld();