why is my APP crashing when I invoke the AsyncTask?

public class Login extends Activity {

    String mail;
    EditText mailIn;    
    Button btSubmit;    

    @Override
    protected void onCreate(Bundle tokenArg) {
        super.onCreate(tokenArg);
        setContentView(R.layout.login);

        mailIn = (EditText)findViewById(R.id.usermail);
        btSubmit = (Button)findViewById(R.id.submit);

        btSubmit.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View thisView) {
                new LoginProc().execute();
            }
        });
    }

    public class LoginProc extends AsyncTask<String, Void, Void> {

        @Override
        protected void onPreExecute() {

            mailIn = (EditText)findViewById(R.id.usermail);
            mail = mailIn.getText().toString();

            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(String... params) {
            Toast.makeText(getApplicationContext(), mail, Toast.LENGTH_SHORT).show();
            return null;
        }
    }
}

I'm trying to make the String name get it's value on the preExecute method, but it happens that the app crashes on that point. Even if I take the preExecute and do that on the doInBrackground, it still crashes. What's wrong?