Android Recycler的Adapter的数据List<>中有null值时，调用notifyDataSetChanged视图数据不改变，但是adapter中的list值已经改变了

Question

Android Recycler的Adapter的数据List<>中有null值时，调用notifyDataSetChanged视图数据不改变，但是adapter中的list值已经改变了
但是在setAdapter的时候有null值可以正常显示。求解释

 provinceSelected.observe(requireActivity(), Observer {
            schoolAdapter.updateDataList(getUniversityProvinced())
            schoolAdapter.notifyDataSetChanged()
        })

public void updateDataList(List<T> list){
        mDataList.clear();
        mDataList.addAll(list);
    }

private fun getUniversityProvinced(): MutableList<University?>{
        val provinceNames = mutableListOf<University?>()
        provinceNames.add(null)
        val zone = provinceSelected.value?.id
        provincesAndUniversities.universities?.let {
            for (i in it){
                if(zone == i.zone){
                    provinceNames.add(i)
                }
            }
        }
        provinceNames.add(null)
        return provinceNames
    }