Agenda has stopped

Ao tentar inserir um novo aluno, no memento de salvar o aplicativo para, só consigo editar existentes.

package br.com.alura.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.widget.Toast;

import br.com.alura.agenda.dao.AlunoDAO;
import br.com.alura.agenda.modelo.Aluno;

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);
        Intent intent = getIntent();
        Aluno aluno = (Aluno) intent.getSerializableExtra("aluno");
        if (aluno != null) {
            helper.preencheFormulario(aluno);
        }


    }

    @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_formulario_ok:
                Aluno aluno = helper.pegaAluno();

                AlunoDAO dao = new AlunoDAO(this);
                if (aluno.getId() != null){
                    dao.altera(aluno);
                } else {
                    dao.insere(aluno);
                }
                dao.close();


                Toast.makeText(FormularioActivity.this, "Aluno " + aluno.getNome() + " salvo!", Toast.LENGTH_SHORT).show();

                finish();
                break;
        }

        return super.onOptionsItemSelected(item);
    }
}

package br.com.alura.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 android.support.annotation.NonNull;

import java.util.ArrayList;
import java.util.List;
import br.com.alura.agenda.modelo.Aluno;

/**
 * Created by julio.maggieri on 24/01/2018.
 */

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        String sql = "CREATE TABLE Alunos (id INTEGER PRIMARY KEY, nome TEXT NOT NULL, endereco TEXT, telefone TEXT, site TEXT, nota REAL)";
        db.execSQL(sql);
    }

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

    public void insere(Aluno aluno) {
        SQLiteDatabase db = getWritableDatabase();

        ContentValues dados = pegaDadosDoAluno(aluno);

        db.insert("Alunos", null, dados);
    }

    @NonNull
    private ContentValues pegaDadosDoAluno(Aluno aluno) {
        ContentValues dados = new ContentValues();
        dados.put("nome", aluno.getNome());
        dados.put("endereco", aluno.getEndereco());
        dados.put("telefone", aluno.getTelefone());
        dados.put("site", aluno.getSite());
        dados.put("nota", aluno.getNota());
        return dados;
    }

    public List<Aluno> buscaAlunos() {
        String sql = "SELECT * from Alunos;";
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.rawQuery(sql, null);

        List<Aluno> alunos = new ArrayList<>();
        while(c.moveToNext()){
            Aluno aluno = new Aluno();
            aluno.setId(c.getLong(c.getColumnIndex("id")));
            aluno.setNome(c.getString(c.getColumnIndex("nome")));
            aluno.setEndereco(c.getString(c.getColumnIndex("endereco")));
            aluno.setTelefone(c.getString(c.getColumnIndex("telefone")));
            aluno.setSite(c.getString(c.getColumnIndex("site")));
            aluno.setNota(c.getDouble(c.getColumnIndex("nota")));
            alunos.add(aluno);
        }
        c.close();

        return alunos;
    }

    public void deleta(Aluno aluno) {
        SQLiteDatabase db = getWritableDatabase();
        String[] params = {aluno.getId().toString()};
        db.delete("Alunos", "id = ?", params);
    }

    public void altera(Aluno aluno) {
        SQLiteDatabase db = getWritableDatabase();

        ContentValues dados = pegaDadosDoAluno(aluno);

        String[] params = {aluno.getId().toString()};
        db.update("Alunos", dados, "id = ?", params);
    }
}