How to insert a list of Cloud Firesotre into an ExpandableListView

How can I enter data obtained from Cloud Firestore in a ExpandableListView ?

I have a "places" document in a "Categories" collection.

ThedocumenthasaVector,alsocalled"places", with 5 Strings . The goal is to get these Strings and insert them into the ExpandableListView as headers.

But I'm having trouble converting the Map<String, Object> object that comes from the DocumentSnapshot, to insert it in the list of headers.

MainActivity.java

package com.rtech.denilsonrenato.expandablelistview;

import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import android.util.Log;

import android.widget.ExpandableListView;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;

import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.FirebaseFirestore;

public class MainActivity extends AppCompatActivity {

    ExpandableListAdapter listAdapter;
    ExpandableListView expListView;
    List<String> listDataHeader;
    HashMap<String, List<String>> listDataChild;
    private static final String TAG = "WORKED";
    private static ArrayList<Type> mArrayList = new ArrayList<>();

    private DocumentReference docRef1 = FirebaseFirestore.getInstance().document("Categorias/places");

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        expListView = findViewById(R.id.lvExp);
        prepareListData();
        listAdapter = new ExpandableListAdapter(this, listDataHeader, listDataChild);
        expListView.setAdapter(listAdapter);

    }
    private void prepareListData() {
        listDataHeader = new ArrayList<String>();
        listDataChild = new HashMap<String, List<String>>();



        docRef1.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                if (task.isSuccessful()){
                    DocumentSnapshot documentSnapshot = task.getResult();
                    if (documentSnapshot != null) {
                    //STUCK HERE DOWNWARDS!!!//
                        Map<String,Object> data = documentSnapshot.getData();

                        List<String> list = new ArrayList<String>(data.values());

                       for (int i=0;i<data.size();i++){

                           listDataHeader.add();

                            listDataChild.put(listDataHeader)
                        }
                        Log.i(TAG, "DocumentSnapshot data: " + data);
                          //THE LOGS ARE DISPLAYING THE DATA FINE
                    } else {
                        Log.i(TAG, "No such document");
                    }
                } else {
                    Log.i(TAG, "get failed with ", task.getException());
                }
            }
        });


    }
}

ExpandableListAdapter.java:

package com.rtech.denilsonrenato.expandablelistview;

import android.content.Context;
import android.graphics.Typeface;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.TextView;

import java.util.HashMap;
import java.util.List;

/**
 * Created by DenilsonRenato on 12/19/2017.
 */

public class ExpandableListAdapter extends BaseExpandableListAdapter {

    private Context _context;
    private List<String> _listDataHeader;
    private HashMap<String, List<String>> _listDataChild;

    public ExpandableListAdapter(Context context, List<String> listDataHeader, HashMap<String, List<String>> listDataChild) {
        _context = context;
        _listDataHeader = listDataHeader;
        _listDataChild = listDataChild;
    }


    @Override
    public Object getChild(int groupPosition, int childPosititon) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .get(childPosititon);
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public View getChildView(int groupPosition, final int childPosition,
                             boolean isLastChild, View convertView, ViewGroup parent) {

        final String childText = (String) getChild(groupPosition, childPosition);

        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.list_item, null);
        }

        TextView txtListChild = (TextView) convertView
                .findViewById(R.id.lblListItem);

        txtListChild.setText(childText);
        return convertView;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return this._listDataHeader.get(groupPosition);
    }

    @Override
    public int getGroupCount() {
        return this._listDataHeader.size();
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded,
                             View convertView, ViewGroup parent) {
        String headerTitle = (String) getGroup(groupPosition);
        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.list_group, null);
        }

        TextView lblListHeader = (TextView) convertView
                .findViewById(R.id.lblListHeader);
        lblListHeader.setTypeface(null, Typeface.BOLD);
        lblListHeader.setText(headerTitle);

        return convertView;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }
}