Problems with Android Studio + Firebase

Well I'm developing this app, to be able to display the information of a user in an arraylist, I execute the code and it simply locks and closes the app, I reviewed the code and did not find anything, does anyone know where find the error?

It shows this error:

 java.lang.RuntimeException: Unable to start activity 
 ComponentInfo{com.example.matheus.kypyy/com.example.matheus.kypyy.MainActivity}:
 java.lang.NullPointerException: Attempt to invoke virtual method 
'java.util.ArrayList com.example.matheus.kypyy.FireHelper.retorno()' on a 
 null object reference

Thank you in advance ...

// User Class

public class Usuarios {

private String nome;
private String Data;
private String Rg;
private String Cpf;
private String Endereco;
private String Doenca;
private String Profissao;

public void Usuarios(){

}

public String getCpf() {
    return Cpf;
}

public void setCpf(String cpf) {
    Cpf = cpf;
}

public String getData() {
    return Data;
}

public void setData(String data) {
    Data = data;
}

public String getDoenca() {
    return Doenca;
}

public void setDoenca(String doenca) {
    Doenca = doenca;
}

public String getEndereco() {
    return Endereco;
}

public void setEndereco(String endereco) {
    Endereco = endereco;
}

public String getNome() {
    return nome;
}

public void setNome(String nome) {
    this.nome = nome;
}

public String getProfissao() {
    return Profissao;
}

public void setProfissao(String profissao) {
    Profissao = profissao;
}

public String getRg() {
    return Rg;
}

public void setRg(String rg) {
    Rg = rg;
 }
}

// BD Firebase class

public class FireHelper {
DatabaseReference bd;
Boolean saved;
ArrayList<Usuarios> usuarioss = new ArrayList<>();

public FireHelper(DatabaseReference bd) {
    this.bd = bd;
}
public Boolean save(Usuarios usuarios){
    if(usuarios == null){
        saved = false;
    }
    else
    {
        try{
            bd.child("Usuarios").push().setValue(usuarios);
        }catch (DatabaseException e){
            e.printStackTrace();
            saved = false;
        }
    }
    return saved;
}

private void fetchData(DataSnapshot dataSnapshot){
    usuarioss.clear();

    for(DataSnapshot ds : dataSnapshot.getChildren()){
        Usuarios usuarios = ds.getValue(Usuarios.class);
        usuarioss.add(usuarios);
    }
}

public ArrayList<Usuarios> retorno(){
    bd.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            fetchData(dataSnapshot);
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            fetchData(dataSnapshot);
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    return usuarioss;
 }
}

// Main class where you will display the data

 public class MainActivity extends AppCompatActivity {
 DatabaseReference bd;
 FireHelper helper;
 AdapterCustom adapter;
 ListView lv;
 EditText      textViewNome,textViewData,textViewRg,
 textViewCpf,textViewEndereco,textViewDoenca,textViewProfissao;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    //layout
    lv = (ListView) findViewById(R.id.lv);
    //Adaptador
    adapter = new AdapterCustom(this, helper.retorno());
    lv.setAdapter(adapter);
    //Banco de dados
    bd = FirebaseDatabase.getInstance().getReference();
    helper = new FireHelper(bd);

    FloatingActionButton fab = (FloatingActionButton)findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view){
        displayInputDialog();}
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

private void displayInputDialog(){
    Dialog d = new Dialog(this);
    d.setTitle("Savo no banco de dados");
    d.setContentView(R.layout.input_dialog);

    //Declarando objetos
    final TextView textViewNome =    
    (TextView)d.findViewById(R.id.editTextNome);
    final TextView textViewData = 
    (TextView)d.findViewById(R.id.editTextData);
    final TextView textViewRg = (TextView)d.findViewById(R.id.editTextRg);
    final TextView textViewCpf = (TextView)d.findViewById(R.id.editTextCpf);
    final TextView textViewEndereco = 
    (TextView)d.findViewById(R.id.editTextEndereco);
    final TextView textViewDoenca = 
    (TextView)d.findViewById(R.id.editTextDoenca);
    final TextView textViewProfissao = 
    (TextView)d.findViewById(R.id.editTextProfissao);

    Button saveButton = (Button)d.findViewById(R.id.savebutton);



    //Salvar...
    saveButton.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            String nome = textViewNome.getText().toString();
            String data = textViewData.getText().toString();
            String rg = textViewRg.getText().toString();
            String cpf = textViewCpf.getText().toString();
            String endereco = textViewEndereco.getText().toString();
            String doenca = textViewDoenca.getText().toString();
            String profissao = textViewProfissao.getText().toString();
            Usuarios u = new Usuarios();
            u.setNome(nome);
            u.setData(data);
            u.setRg(rg);
            u.setCpf(cpf);
            u.setEndereco(endereco);
            u.setDoenca(doenca);
            u.setProfissao(profissao);

            if(nome != null && nome.length()> 0){
                if(helper.save(u)){
                    textViewNome.setText("");
                    textViewData.setText("");
                    textViewRg.setText("");
                    textViewCpf.setText("");
                    textViewEndereco.setText("");
                    textViewDoenca.setText("");
                    textViewProfissao.setText("");

                    adapter = new AdapterCustom(MainActivity.this, 
           helper.retorno());
                    lv.setAdapter(adapter);

                }else {
                    Toast.makeText(MainActivity.this, "sem dados", 
               Toast.LENGTH_SHORT).show();
                }
            }
        }});
        }

        }

// Adapter Class

public class AdapterCustom extends BaseAdapter {
Context c;
ArrayList<Usuarios> usuarioss;

public AdapterCustom(Context c, ArrayList<Usuarios> usuarioss) {
    this.c = c;
    this.usuarioss = usuarioss;
}

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

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

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

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    if(convertView== null){
        convertView = LayoutInflater.from(c).inflate(R.layout.model,parent,false);
    }
    //Declarando objetos
    TextView textViewNome = (TextView)convertView.findViewById(R.id.textViewNome);
    TextView textViewData = (TextView)convertView.findViewById(R.id.textViewData);
    TextView textViewRg = (TextView)convertView.findViewById(R.id.textViewRg);
    TextView textViewCpf = (TextView)convertView.findViewById(R.id.textViewCpf);
    TextView textViewEndereco = (TextView)convertView.findViewById(R.id.textViewEndereco);
    TextView textViewDoenca = (TextView)convertView.findViewById(R.id.textViewDoenca);
    TextView textViewProfissao = (TextView)convertView.findViewById(R.id.textViewProfissao);

    final Usuarios u = (Usuarios) this.getItem(position);

    textViewNome.setText(u.getNome());
    textViewData.setText(u.getData());
    textViewRg.setText(u.getRg());
    textViewCpf.setText(u.getCpf());
    textViewEndereco.setText(u.getEndereco());
    textViewDoenca.setText(u.getDoenca());
    textViewProfissao.setText(u.getProfissao());

    //Função onclick

    convertView.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Toast.makeText(c, u.getNome(), Toast.LENGTH_SHORT).show();
        }
    });



    return convertView;
 }
}