我正在尝试使用firebase，但当尝试在listView中加载数据时，应用程序返回错误：

java.lang.RuntimeException:无法启动activity ComponentInfo {my activity} java.lang.NullPointerException:尝试对 空对象引用调用虚方法'void com.firebase.ui.database.FirebaseListAdapter.startListening ()‘。

如果有人能看到这个问题，那将是非常有帮助的。

我的数据库的方面：

location
   40434734
      latitude: 40.434734
      longitude: -3.70701
      txt: "test"

我的班级：

public class start extends AppCompatActivity{
ListView lv;
FirebaseListAdapter adapter;
@Override
public void onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) {
    super.onCreate(savedInstanceState, persistentState);
    lv = (ListView) findViewById(R.id.listView);
    Query query = FirebaseDatabase.getInstance().getReference().child("location");
    FirebaseListOptions<LocationData> options = new FirebaseListOptions.Builder<LocationData>()
            .setLayout(R.layout.list_location)
            .setQuery(query,LocationData.class)
            .build();
    adapter = new FirebaseListAdapter(options) {
        @Override
        protected void populateView(View v, Object model, int position) {
            TextView stdID = v.findViewById(R.id.nombre);
            LocationData  ld = (LocationData) model;
            stdID.setText(ld.getTxt().toString());
    lv.setAdapter(adapter);
@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
@Override
protected void onStop() {
    super.onStop();
    adapter.stopListening();
}

}

创建对象的LocationData类：

public class LocationData {
double latitude;
double longitude;
String txt;
public LocationData(){
public LocationData(double latitude, double longitude, String txt) {
    this.latitude = latitude;
    this.longitude = longitude;
    this.txt = txt;
public String getTxt() {
    return txt;
public void setTxt(String txt) {
    this.txt = txt;
public double getLatitude() {
    return latitude;
public void setLatitude(double latitude) {
    this.latitude = latitude;
public double getLongitude() {
    return longitude;
public void setLongitude(double longitude) {