Android内部存储无法创建文件，是不是因为开头fos定义成null，如果是的话要怎么修改

package com.example.test;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Map;

public class RegisterActivity extends AppCompatActivity {

private Button register;
private Button cancel;
private EditText etname;
private EditText etpassword;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_register);

    register = findViewById(R.id.btn_register);
    cancel = findViewById(R.id.btn_cancel);
    etname = findViewById(R.id.et_name);
    etpassword = findViewById(R.id.et_password);


    register.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            FileOutputStream fos = null;//定义文件输出流
            String name = etname.getText().toString().trim();
            String pwd = etpassword.getText().toString().trim();
            try {
                fos = openFileOutput("message.txt", MODE_APPEND);//追加新注册用户信息
                fos.write(name.getBytes());
                fos.write(pwd.getBytes());
                fos.flush();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (fos != null) {
                    try {
                        fos.close();
                        Toast.makeText(RegisterActivity.this, "注册成功", Toast.LENGTH_SHORT).show();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                //页面跳转延迟线程
                new Thread() {
                    public void run() {
                        super.run();
                        try {
                            Thread.sleep(500);
                            Intent i = new Intent(RegisterActivity.this, MainActivity.class);
                            startActivity(i);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }.start();

                Toast.makeText(RegisterActivity.this, "注册成功", Toast.LENGTH_SHORT).show();
                return;
            }
        }
    });

    cancel.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent i = new Intent(RegisterActivity.this, MainActivity.class);
            startActivity(i);
        }
    });
    }
}

并不是,需要插入状态量