问应用启动时CustomAdapter为空

EN

class CustomAdapter extends BaseAdapter {

private Context c;
private ArrayList<Item> items;


 CustomAdapter(Context c, ArrayList<Item> items) {
    super();
    this.c = c;
    this.items = items;
    Log.e("Hee",items.size()+"");
}

@Override
public int getCount() {
    return items.size();
}

@Override
public Object getItem(int position) {
    return items.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, final ViewGroup parent) {

    LayoutInflater inflater = (LayoutInflater) parent.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    if(convertView==null)
    {
        convertView= inflater.inflate(R.layout.model,parent,false);
    }
    final TextView titleTxt= (TextView) convertView.findViewById(R.id.titleTxt);
    TextView priceTxt= (TextView) convertView.findViewById(R.id.priceTxt);
    TextView descTxt= (TextView) convertView.findViewById(R.id.descTxt);
    ImageView image = (ImageView) convertView.findViewById(imageView);

    final Item s = (Item) this.getItem(position);

    titleTxt.setText(s.getSale());
    priceTxt.setText(s.getPrice()+"€");
    descTxt.setText(s.getDescription());
    String url = s.getUrl();

    if(url != null){
        Glide.with(c).load(url).into(image);
    }else{
        Glide.with(c).load("http://www.video-furn.com/images/icons/AVFI-Image-not-available-icon.jpg").into(image);
    }

    //ONITECLICK
    convertView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            String title = "Select option";
            CharSequence [] itemlist = {"Chiama", "Nuovo messaggio",};

            AlertDialog.Builder builder = new AlertDialog.Builder(c);
            builder.setTitle(title);
            builder.setItems(itemlist, new DialogInterface.OnClickListener(){

                @Override
                public void onClick(DialogInterface dialog, int witch) {
                    switch(witch){
                        case 0:
                            Intent phone_intent = new Intent(Intent.ACTION_DIAL);
                            phone_intent.setData(Uri.parse("tel:"+s.getPhone()));
                            c.startActivity(phone_intent);
                            break;

                        case 1:
                            Intent send_intent = new Intent(Intent.ACTION_VIEW);
                            send_intent.setData(Uri.parse("sms:"+s.getPhone()));
                            c.startActivity(send_intent);
                            break;


                        default:
                            break;
                    }

                }
            });

            AlertDialog alert = builder.create();
            alert.setCancelable(true);
            alert.show();

        }
    });

    return convertView;
}}

public class MainActivity_Fragment extends Fragment {

private DatabaseReference mDatabase;
private FirebaseHelper helper;
private ListView listView;
private CustomAdapter adapter;
private View view;
private Snackbar snackbar;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    mDatabase = FirebaseDatabase.getInstance().getReference();
    helper = new FirebaseHelper(mDatabase);

    view = inflater.inflate(R.layout.activity_main_fragment, container, false);
    adapter = new CustomAdapter(getActivity(), helper.retrieve());

    FloatingActionButton fabButton = (FloatingActionButton) view.findViewById(R.id.fabButton);

    listView = (ListView) view.findViewById(R.id.listview);

    listView.setAdapter(adapter);
    listView.setVisibility(View.VISIBLE);

    connectionState();

    fabButton.setOnClickListener(new View.OnClickListener(){

        @Override
        public void onClick(View v) {
         addItem();
        }
    });

    Log.e("list adapter--",""+adapter.getCount());

    return view; }