Fixed visibility of respawned objects

The OldBucket has to be null before adding it to the bucket of the map.
Otherwise, the players ObserverToWorldViewAdapter will return early,
because it thinks it already observes the oldBucket (='where the object
came from').
See also #39.

src/GameLogic/BucketAreaOfInterestManager.cs

@@ -30,13 +30,15 @@ public BucketAreaOfInterestManager(int chunkSize)
         /// <inheritdoc/>
         public void AddObject(ILocateable obj)
         {
-            this.Map[obj.X, obj.Y].Add(obj);
+            var newBucket = this.Map[obj.X, obj.Y];
             if (obj is IHasBucketInformation bucketInfo)
             {
                 bucketInfo.OldBucket = null;
-                bucketInfo.NewBucket = this.Map[obj.X, obj.Y];
+                bucketInfo.NewBucket = newBucket;
             }
 
+            newBucket.Add(obj);
+
             if (obj is IBucketMapObserver observingPlayer)
             {
                 this.UpdateObservingBuckets(obj.X, obj.Y, observingPlayer);