Update IslandLevelCalculator.java (#314)

Fix for occasional errors likely due to the use of the remove() method within a lambda expression inside the thenAccept method. This lambda expression is executed asynchronously, which means that the iterator may not be in a consistent state when remove() is called.

src/main/java/world/bentobox/level/calculators/IslandLevelCalculator.java

@@ -639,6 +639,7 @@ public class IslandLevelCalculator {
 
     private void handleStackedBlocks() {
     // Deal with any stacked blocks
+    List<Location> toRemove = new ArrayList<>();
     Iterator<Location> it = stackedBlocks.iterator();
     while (it.hasNext()) {
         Location v = it.next();
@@ -657,8 +658,17 @@ public class IslandLevelCalculator {
                     checkBlock(stackedBlock.getType(), belowSeaLevel);
                 }
             }
-		it.remove();
+            synchronized (toRemove) {
+                toRemove.add(v);
+            }
         });
     }
+
+    // Wait for all asynchronous tasks to complete before removing elements
+    // Remove the elements collected in toRemove
+    synchronized (toRemove) {
+        stackedBlocks.removeAll(toRemove);
     }
 }
+
+}