为什么在Activity中访问片段的TextView会引发错误

由于

fo

FragOne fo;@Overrideprotected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);    setSupportActionBar(toolbar);    fo.setTextViewText("This is added from Activity");    ...}

fo.setTextViewText()

NPE