从父活动传递字符串时，片段中的空对象引用

Question

这向我显示了空指针异常：

if (!bundle.getString("username").equals("start")) {

因为我无法获得所需的字符串。从字面上的最后几个小时，我真的受够了。请帮我解决一下这个。谢谢。

家庭活动（Java文件）

public class HomeActivity extends AppCompatActivity {
TabLayout tabLayout;
ViewPager viewPager;
ViewPagerAdapter viewPagerAdapter;
private boolean isUserClickedBackButton = false;
Handler handler = new Handler();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);
    Toolbar toolbar = findViewById(R.id.action_bar);
    toolbar.setTitleTextColor(getResources().getColor(R.color.black));
    setSupportActionBar(toolbar);
    tabLayout = findViewById(R.id.tabLayout);
    viewPager = findViewById(R.id.viewPager);
    viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
    viewPagerAdapter.addFragments(new EmployerFragment(),"Employer  Login");
    viewPagerAdapter.addFragments(new AdminFragment(),"Admin  Login");
    viewPager.setAdapter(viewPagerAdapter);
    tabLayout.setupWithViewPager(viewPager);
    Bundle bundle = new Bundle();
    bundle.putString("username", getIntent().getExtras().getString("username"));
    bundle.putString("password", getIntent().getExtras().getString("password"));
    EmployerFragment employerFragment = new EmployerFragment();
    employerFragment.setArguments(bundle);
}

@Override
public void onBackPressed() {
    if(!isUserClickedBackButton)
    {
        Toast.makeText(this, "Press back key again to exit !!!", Toast.LENGTH_SHORT).show();
        isUserClickedBackButton=true;
        handler.postDelayed(new Runnable() {
            @Override
            public void run() {
                isUserClickedBackButton=false;
            }
        },3000);
    }
    else
    {
        finish();
        System.exit(0);
    }
}

}

雇主片段（Java文件）

public class EmployerFragment extends Fragment {
EditText user, pass;
Button login, register;
View view;

public EmployerFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.fragment_employer, container, false);
    login = view.findViewById(R.id.loginEmp);
    register = view.findViewById(R.id.registerEmp);
    user = view.findViewById(R.id.userEmp);
    pass = view.findViewById(R.id.passEmp);
    Bundle bundle = getArguments();

    if (!bundle.getString("username").equals("start")) {
        user.setText(bundle.getString("username"));
        pass.setText(bundle.getString("password"));
    }

    login.requestFocus();

    login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent administrator = new Intent(getActivity(), EmployerPanel.class);
            if (user.getText().toString().equals("")) {
                Toast.makeText(getActivity(), "Please enter username", Toast.LENGTH_SHORT).show();
                user.requestFocus();
            } else if (pass.getText().toString().equals("")) {
                Toast.makeText(getActivity(), "Please enter password", Toast.LENGTH_SHORT).show();
                pass.requestFocus();
            } else if (user.getText().toString().equals("Employer") && pass.getText().toString().equals("employer")) {
                startActivity(administrator);
                getActivity().finish();
            } else {
                user.setText("");
                pass.setText("");
                Toast.makeText(getActivity(), "Invalid Username or Password", Toast.LENGTH_LONG).show();
                user.requestFocus();
            }
        }
    });

    register.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent register = new Intent(getActivity(), RegisterEmployer.class);
            startActivity(register);
            getActivity().finish();
        }
    });

    return view;
}

}