Issue 6366 - Elemhide thread is accessing released engine

libadblockplus-android/src/org/adblockplus/libadblockplus/android/SingleInstanceEngineProvider.java

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 40 matching lines @@
   private boolean developmentBuild;
   private String preloadedPreferenceName;
   private Map<String, Integer> urlToResourceIdMap;
   private AdblockEngine engine;
   private CountDownLatch engineCreated;
   private Long v8IsolateProviderPtr;
   private List<EngineCreatedListener> engineCreatedListeners =
     new LinkedList<EngineCreatedListener>();
   private List<EngineDisposedListener> engineDisposedListeners =
     new LinkedList<EngineDisposedListener>();
-  private Object engineLock = new Object();
+  private final Object engineLock = new Object();
 
   /*
     Simple ARC management for AdblockEngine
     Use `retain` and `release`
    */
 
   private AtomicInteger referenceCounter = new AtomicInteger(0);
 
   /**
    * Init with context
@@ skipping 225 matching lines @@
   {
     return referenceCounter.get();
   }
 
   @Override
   public Object getEngineLock()
   {
     return engineLock;
   }
 }