DatePicker di Android Studio

FormMahasiswa.java

package com.susantokun.project_1;

import android.app.DatePickerDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.Spinner;
import android.widget.TextView;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Locale;

public class FormMahasiswa extends AppCompatActivity {

    private DatePickerDialog datePickerDialog;
    private SimpleDateFormat dateFormatter;
    private TextView tvDateResult;
    private EditText btDatePicker;

    Button bsimpan;
    EditText enpm, enama, etempatlahir;
    TextView thasil;
    RadioGroup rgjk;
    Spinner skelas, sagama;

    private void showDateDialog(){
        Calendar newCalendar = Calendar.getInstance();
        datePickerDialog = new DatePickerDialog(this, new DatePickerDialog.OnDateSetListener() {

            @Override
            public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {

                Calendar newDate = Calendar.getInstance();
                newDate.set(year, monthOfYear, dayOfMonth);

                tvDateResult.setText(dateFormatter.format(newDate.getTime()));
            }

        },newCalendar.get(Calendar.YEAR), newCalendar.get(Calendar.MONTH), newCalendar.get(Calendar.DAY_OF_MONTH));

        datePickerDialog.show();
    }

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

        // inisialisasi
        dateFormatter = new SimpleDateFormat("dd-MM-yyyy", Locale.US);
        tvDateResult = (TextView) findViewById(R.id.tv_dateresult);
        btDatePicker = (EditText) findViewById(R.id.tv_dateresult);

        bsimpan = (Button) findViewById(R.id.simpan);
        enpm = (EditText) findViewById(R.id.isinpm);
        enama = (EditText) findViewById(R.id.isinama);
        thasil = (TextView) findViewById(R.id.hasil);
        rgjk = (RadioGroup) findViewById(R.id.jk);
        skelas = (Spinner) findViewById(R.id.kelas);
        sagama  = (Spinner) findViewById(R.id.agama);
        etempatlahir = (EditText) findViewById(R.id.tempatlahir);

        bsimpan.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String inputnpm = String.valueOf(enpm.getText().toString());
                String inputnama = String.valueOf(enama.getText().toString());
                String inputtempatlahir = String.valueOf(etempatlahir.getText().toString());
                String inputotomatis = String.valueOf(tvDateResult.getText().toString());

                int gender = rgjk.getCheckedRadioButtonId();
                RadioButton jk = (RadioButton) findViewById(gender);
                String inputjk = String.valueOf(jk.getText().toString());

                thasil.setText("\n" + "NPM\t\t\t\t\t\t\t\t\t\t\t: " + inputnpm + "\n" +
                        "Nama\t\t\t\t\t\t\t\t\t\t: " + inputnama + "\n" +
                        "Jenis Kelamin\t\t: " + inputjk + "\n" +
                        "Kelas\t\t\t\t\t\t\t\t\t\t: " + skelas.getSelectedItem().toString() + "\n" +
                        "Agama\t\t\t\t\t\t\t\t\t: " + sagama.getSelectedItem().toString() + "\n" +
                        "Jenis Kelamin\t\t: " + inputtempatlahir + "\n" +
                        "Tanggal Lahir\t\t\t: " + inputotomatis + "\n");
            }
        });

        dateFormatter = new SimpleDateFormat("dd-MM-yyyy", Locale.US);
        tvDateResult = (TextView) findViewById(R.id.tv_dateresult);
        btDatePicker = (EditText) findViewById(R.id.tv_dateresult);
        btDatePicker.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showDateDialog();
            }
        });

    }
}