Synchronise access to the PriorityQueue used for effect queuing in AbstractWorld

- Makes it thread-safe

worldedit-core/src/main/java/com/sk89q/worldedit/world/AbstractWorld.java

@@ -111,6 +111,8 @@ public abstract class AbstractWorld implements World {
     public boolean queueBlockBreakEffect(Platform server, BlockVector3 position, BlockType blockType, double priority) {
         if (taskId == -1) {
             taskId = server.schedule(0, 1, () -> {
+                //FAWE start - access to PriorityQueue is not thread-safe
+                synchronized (effectQueue) {
                     int max = Math.max(1, Math.min(30, effectQueue.size() / 3));
                     for (int i = 0; i < max; ++i) {
                         if (effectQueue.isEmpty()) {
@@ -119,6 +121,8 @@ public abstract class AbstractWorld implements World {
 
                         effectQueue.poll().play();
                     }
+                }
+                //FAWE end
             });
         }
 
@@ -126,7 +130,11 @@ public abstract class AbstractWorld implements World {
             return false;
         }
 
+        //FAWE start - access to PriorityQueue is not thread-safe
+        synchronized (effectQueue) {
             effectQueue.offer(new QueuedEffect(position.toVector3(), blockType, priority));
+        }
+        //FAWE end
 
         return true;
     }