Fix slow AiPackage::getTarget calls

Assume there are no cell refs with empty ref id.

CHANGELOG.md

@@ -143,6 +143,7 @@
     Bug #6036: OpenMW-CS: Terrain selection at the border of cells omits certain corner vertices
     Bug #6043: Actor can have torch missing when torch animation is played
     Bug #6047: Mouse bindings can be triggered during save loading
+    Bug #6136: Game freezes when NPCs try to open doors that are about to be closed
     Feature #390: 3rd person look "over the shoulder"
     Feature #832: OpenMW-CS: Handle deleted references
     Feature #1536: Show more information about level on menu

apps/openmw/mwmechanics/aipackage.cpp

@@ -55,6 +55,11 @@ MWWorld::Ptr MWMechanics::AiPackage::getTarget() const
 
     if (mTargetActorId == -1)
     {
+        if (mTargetActorRefId.empty())
+        {
+            mTargetActorId = -2;
+            return MWWorld::Ptr();
+        }
         MWWorld::Ptr target = MWBase::Environment::get().getWorld()->searchPtr(mTargetActorRefId, false);
         if (target.isEmpty())
         {