Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean androidx.fragment.app.FragmentManager.isDestroyed()' on a null object reference

2015年的老项目，升级到androidX下，遇到的这样的问题，

找到fragment的基类 onDetach方法，注释掉下面的代码 try...catch里面的反射方法代码

@Override
public void onDetach() {
   super.onDetach();
   try {
      Field childFragmentManager = Fragment.class.getDeclaredField("mChildFragmentManager");
      childFragmentManager.setAccessible(true);
      childFragmentManager.set(this, null);

   } catch (NoSuchFieldException e) {
      throw new RuntimeException(e);
   } catch (IllegalAccessException e) {
      throw new RuntimeException(e);
   }
}