Fix concurrent errors in URLClassLoaderAccess (#3403)

common/src/main/java/me/lucko/luckperms/common/plugin/classpath/URLClassLoaderAccess.java

@@ -167,10 +167,12 @@ public abstract class URLClassLoaderAccess {
                 URLClassLoaderAccess.throwError(new NullPointerException("unopenedURLs or pathURLs"));
             }
 
+            synchronized (this.unopenedURLs)  {
                 this.unopenedURLs.add(url);
                 this.pathURLs.add(url);
             }
         }
+    }
 
     private static class Noop extends URLClassLoaderAccess {
         private static final Noop INSTANCE = new Noop();