viewdisappear后调用组件的方法容易crash

[jqoo]

`

• (SEL)_methodWithComponetName:(NSString *)name withMethod:(NSString *)method isSync:(BOOL *)isSync
  {
  WXAssert(name && method, @"Fail to find selector with module name and method, please check if the parameters are correct ！");
  //...
  }
  闪退在了这里，原因是在页面pop时，发出了disappear，然后销毁了页面里的实例，但页面里disappear事件的回调方法里，调用组件的方法时，self.instance依然是非空，根据ref找不到组件：
• (void)invoke
  {
  WXPerformBlockOnComponentThread(^{
  WXComponent *component = [self.instance componentForRef:_componentRef];
  if (!component) {
  WXLogError(@"component not found for ref:%@, type:%@", _componentRef, _componentName);
  }
  //...
  }
  `

[YorkShen]

You just find the answer by yourself, great work.

You could give us a PR if you can, I am very happy to talk with you implemenation detail or review you PR in mailing list

[YorkShen]

Duplicated with apache/incubator-weex#2922