Prevent concurrent modification exception when moving token to a zone besides the graveyard

forge-core/src/main/java/forge/util/FCollection.java

@@ -251,4 +251,10 @@ public class FCollection<T> implements List<T>, Set<T>, FCollectionView<T>, Clon
         }
         return subList;
     }
+
+    @Override
+    public Iterable<T> threadSafeIterator() {
+        //create a new linked list for iterating to make it thread safe and avoid concurrent modification exceptions
+        return new LinkedList<T>(list);
+    }
 }

forge-core/src/main/java/forge/util/FCollectionView.java

@@ -13,4 +13,5 @@ public interface FCollectionView<T> extends Iterable<T> {
     int lastIndexOf(Object o);
     boolean contains(Object o);
     List<T> subList(int fromIndex, int toIndex);
+    Iterable<T> threadSafeIterator();
 }