popViewControllerAnimated 引起崩溃 找不到原因 (无报错信息)

if([NSThreadisMainThread]) {
   NSLog(@"Yay!"); 
   [self.navigationController popViewControllerAnimated:YES];
}else{
 NSLog(@"Humph, switching to main");
 dispatch_async(dispatch_get_main_queue(),  ^{      
  [self.navigationController popViewControllerAnimated:YES];   
 });
}