android-studio - MaterialDatePicker 在 Android 中不起作用

Question

我正在尝试使用在 Internet 上找到的代码，但它根本没有运行。满足 bulid.gradle 条件并且代码没有问题。当我单击 Martial DatePicker 的按钮时，应用程序退出。

//梯度

implementation 'com.google.android.material:material:1.3.0'

//activity_add_plan

<Button
        android:id="@+id/mdBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="material datepicker"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="40dp"/>

//添加PlanActivity

public class AddPlanActivity extends AppCompatActivity {

private TextView startD, finishD;
private Button mDatepickerBtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add_plan);
    ActionBar actionBar = getSupportActionBar();
    actionBar.hide();

    startD = (TextView)findViewById(R.id.startDay);
    finishD = (TextView)findViewById(R.id.finishDay);
    mDatepickerBtn = findViewById(R.id.mdBtn);

    //Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
    //cal.clear();

    //final long today = MaterialDatePicker.todayInUtcMilliseconds();

    MaterialDatePicker.Builder builder = MaterialDatePicker.Builder.datePicker();
    builder.setTitleText("SELECT A DATE");
    //builder.setSelection(today);
    final MaterialDatePicker materialDatePicker = builder.build();

    mDatepickerBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            materialDatePicker.show(getSupportFragmentManager(), "DATE_PICKER");
        }
    });

    materialDatePicker.addOnPositiveButtonClickListener(new MaterialPickerOnPositiveButtonClickListener() {
        @Override
        public void onPositiveButtonClick(Object selection) {
            startD.setText(materialDatePicker.getHeaderText());
        }
    });

}

}