App Agenda parou de funcionar

package com.example.pcpower.agenda.dao;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import com.example.pcpower.agenda.modelo.Cliente;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by PcPower on 22/07/2018.
 */

public class ClienteDAO extends SQLiteOpenHelper {
    public ClienteDAO(Context context) {
        super(context, "Agenda", null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String sql = "CREATE TABLE CLIENTES (id INTEGER PRIMARY KEY, cliente TEXT NOT NULL, largura TEXT, nota REAL);";
        db.execSQL(sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        String sql = "DROP TABLE IF EXISTS Clientes";
        db.execSQL(sql);
        onCreate(db);
    }

    public void insere(Cliente cliente) {
        SQLiteDatabase db = getWritableDatabase();

        ContentValues dados = new ContentValues();
        dados.put("cliente", cliente.getCliente());
        dados.put("largura", cliente.getLargura());
        dados.put("nota", cliente.getNota());

        db.insert("Clientes", null, dados);

    }

    public List<Cliente> buscaclientes() {
        String sql = "SELECT * FROM Clientes;";
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.rawQuery(sql, null);

        List<Cliente> clientes = new ArrayList<Cliente>();
        while (c.moveToNext()) {
            Cliente cliente = new Cliente();
            cliente.setId(c.getLong(c.getColumnIndex("id")));
            cliente.setCliente(c.getString(c.getColumnIndex("cliente")));
            cliente.setLargura(c.getString(c.getColumnIndex("largura")));
            cliente.setNota(c.getDouble(c.getColumnIndex("nota")));
            clientes.add(cliente);
        }

        c.close();
        return null;
    }

}

package com.example.pcpower.agenda.modelo;

/**
 * Created by PcPower on 22/07/2018.
 */

public class Cliente {
    private Long id;
    private String cliente;
    private String largura;
    private Double nota;

    public Double getNota() {
        return nota;
    }

    public void setNota(Double nota) {
        this.nota = nota;
    }

    public String getLargura() {

        return largura;
    }

    public void setLargura(String largura) {
        this.largura = largura;
    }

    public String getCliente() {

        return cliente;
    }

    public void setCliente(String cliente) {
        this.cliente = cliente;
    }

    public Long getId() {

        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return getId() + " - " + getCliente();
    }
}

package com.example.pcpower.agenda;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.example.pcpower.agenda.dao.ClienteDAO;
import com.example.pcpower.agenda.modelo.Cliente;

public class FormularioActivity extends AppCompatActivity {

    private FormularioHelper helper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_formulario);

        helper = new FormularioHelper(this);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu_formulario,menu);

        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()){
            case R.id.menu_ok:
                Cliente cliente = helper.pegacliente();
                ClienteDAO dao = new ClienteDAO(this);
                dao.insere(cliente);
                dao.close();

                Toast.makeText(FormularioActivity.this, "Cliente " + cliente.getCliente()+ " Salvo!", Toast.LENGTH_SHORT).show();

                finish();
                break;
        }
        return super.onOptionsItemSelected(item);
    }
}

Em Resposta coloquei o resto do codigo