在跳过登录活动时获取NullPointerException

Question

public class MainActivity extends Activity {
    Button sub;
    SharedPreferences prefs;
    EditText useret,pwdet;
    String IS_LOGIN = "IsLoggedIn";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if(this.isLoggedIn()==true) {
            checklogin();
        }
        setContentView(R.layout.activity_main);
        useret=(EditText) findViewById(R.id.etuser);
        pwdet=(EditText) findViewById(R.id.etpwd);
        sub=(Button) findViewById(R.id.submit);
        sub.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String username=useret.getText().toString();
                String password=pwdet.getText().toString();
                prefs=getSharedPreferences("codelearn_twitter", MODE_PRIVATE);
                SharedPreferences.Editor editor=prefs.edit();
                editor.putString("key_for_username",username);
                editor.putString("key_for_passwrd",password);
                editor.putBoolean(IS_LOGIN, true);
                editor.commit();

                Intent i=new Intent(MainActivity.this,listactivity.class);
                startActivity(i);
            }
        });
    }
    public void checklogin(){
        if(this.isLoggedIn()==true){
            Intent i=new Intent(MainActivity.this,listactivity.class);
            startActivity(i);
            finish();
        }
    }
    public boolean isLoggedIn(){
        return prefs.getBoolean(IS_LOGIN, false);
    }
}