Fixed multiple LINQ using shared resources and cause crashes

Added an null check in AIObjectiveManager.cs to avoid accessing removed resources
Use shuffledGaps instead of gapList to ensure update order requirement(already in master)
Updated parallelism count

Barotrauma/BarotraumaShared/SharedSource/Map/MapEntity.cs

@@ -816,7 +816,7 @@ namespace Barotrauma
 
             // Gap update (has order dependencies, keep random order but execute sequentially)
             var shuffledGaps = gapList.OrderBy(g => Rand.Int(int.MaxValue)).ToList();
-            Parallel.ForEach(gapList, parallelOptions, gap =>
+            Parallel.ForEach(shuffledGaps, parallelOptions, gap =>
             {
                 PhysicsBodyQueue.IsInParallelContext = true;
                 try