为什么会出现空指针错误

addRoom.setOnClickListener(new android.view.View.OnClickListener(){
    public void onClick(View v) {
        builder = new AlertDialog.Builder(RoomSet.this);
        TableLayout addroom = (TableLayout)getLayoutInflater().inflate(R.layout.addroom, null);
        builder.setView(addroom);
        newRoomET = (EditText)findViewById(R.id.addroom_et);
        builder.setPositiveButton("确定"
                // 为按钮设置监听器
                , new android.content.DialogInterface.OnClickListener()
                {
                    public void onClick(DialogInterface dialog, int which)
                    {
                        //此处可执行登录处理
                        name =  newRoomET.getText().toString();//[b]①处[/b]
                                                ...
                    }
                }
    }
}