Minha Activity fecha ao clicar no botão

package br.com.example.dyanlucas.mangas;



import android.content.Intent;
import android.database.Cursor;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.Toast;
import bancodedados.bancoDados;
import controller.mangacrtl;
import model.Manga;


public class MainActivity extends AppCompatActivity {

    private ListView lstManga;
    private Button novo;
    private Button listar;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        novo = (Button) findViewById(R.id.button6);
        listar = (Button) findViewById(R.id.button7);
        novo.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(MainActivity.this, InsereDado.class);
                startActivity(i);
            }
        });
        listar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(MainActivity.this, consulta.class);
                startActivity(i);
            }
        });
    }

    public class consulta extends AppCompatActivity {
            private ListView lista;
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.consultar_list);

                mangacrtl crud = new mangacrtl(getBaseContext());
                final Cursor cursor = crud.carregarDados();
                String[] nomeCampos = new String[]{bancoDados.D, bancoDados.NOME};
                int[] idViews = new int[]{R.id.idManga, R.id.nomeManga};

                SimpleCursorAdapter adaptador = new SimpleCursorAdapter(getBaseContext(),
                        R.layout.activity_main, cursor, nomeCampos, idViews, 0);
                lista = (ListView) findViewById(R.id.lstManga);
                lista.setAdapter(adaptador);

                lstManga = (ListView) findViewById(R.id.lstManga);
                lstManga.setAdapter(adaptador);
                lstManga.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                        String codigo;
                        cursor.moveToPosition(position);
                        codigo = cursor.getString(cursor.getColumnIndexOrThrow(bancoDados.D));
                        Intent intent = new Intent(MainActivity.this, Alterar.class);
                        intent.putExtra("codigo", codigo);
                        startActivity(intent);
                        finish();

                    }


                });

            }
        }

    public class InsereDado extends AppCompatActivity {

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

            Button botao = (Button) findViewById(R.id.button);

            botao.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    mangacrtl crud = new mangacrtl(getBaseContext());


                    EditText nome = (EditText) findViewById(R.id.editText);
                    EditText descricao = (EditText) findViewById((R.id.editText2));
                    EditText valor = (EditText) findViewById(R.id.editText3);

                    Manga mangas = new Manga();
                    mangas.setNomeManga(nome.getText().toString());
                    mangas.setDecricaoManga(descricao.getText().toString());
                    mangas.setValorManga(Double.parseDouble(valor.getText().toString()));

                    String resultado;

                    resultado = crud.inserirDados(mangas);

                    Toast.makeText(getApplicationContext(), resultado, Toast.LENGTH_LONG).show();
                }
            });

        }
    }

    public class Alterar extends AppCompatActivity {
        EditText nome;
        EditText descricao;
        EditText valor;
        Button alterar;
        Button deletar;
        Cursor cursor;
        mangacrtl crud;
        String codigo;

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

            codigo = this.getIntent().getStringExtra("codigo");

            crud = new mangacrtl(getBaseContext());

             nome = (EditText) findViewById(R.id.editText);
             descricao = (EditText) findViewById((R.id.editText2));
             valor = (EditText) findViewById(R.id.editText3);

            alterar = (Button)findViewById(R.id.button);

            cursor = crud.carregarId(Integer.parseInt(codigo));
            nome.setText(cursor.getString(cursor.getColumnIndexOrThrow(bancoDados.NOME)));
            descricao.setText(cursor.getString(cursor.getColumnIndexOrThrow(bancoDados.DESCRICAO)));
            valor.setText(cursor.getString(cursor.getColumnIndexOrThrow(bancoDados.VALOR)));

            alterar.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {

                    Manga mangas = new Manga();
                    mangas.setNomeManga(nome.getText().toString());
                    mangas.setDecricaoManga(descricao.getText().toString());
                    mangas.setValorManga(Double.parseDouble(valor.getText().toString()));

                    crud.alterarManga(mangas);
                    Intent intent = new Intent(Alterar.this,consulta.class);
                    startActivity(intent);
                    finish();
                }
            });

            deletar.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    crud.excluirManga(Integer.parseInt(codigo));
                    Intent intent = new Intent(Alterar.this,consulta.class);
                    startActivity(intent);
                    finish();
                }
            });

        }
    }









}