При запуске приложения Android происходит сбой из-за java.lang.NullPointerException при onClick

Я хочу сделать так, чтобы пользователь нажал кнопку, чтобы запустить диалог. Он сбрасывается, когда я нажимаю кнопку. Здесь logcat:

nullpointerexception в com.shoppinglist.ShoppingList $1 $2.onClick(ShoppingList.java:50) в android.view.View.performClick(View.java:2344) в android.view.View.onTouchEvent(View.java:4133 ) в android.widget.TextView.onTouchEvent(TextView.java:6510) в android.view.View.dispatchTouchEvent(View.java:3672) в android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:882) на android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:882) в android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:882) в android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:882) в android.view.ViewGroup.dispatchTouchEvent( ViewGroup.java:882) в android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:882)

Вот мой .java файл:

package com.shoppinglist;

import java.util.ArrayList;

import android.app.Dialog;
import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;

public class ShoppingList extends ListActivity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        final EditText et = (EditText) findViewById(R.id.edittext);
        ListView lv = (ListView) findViewById(android.R.id.list);
        final ArrayList<String> items = new ArrayList<String>();
        final ArrayAdapter<String> adapter;
        adapter = new ArrayAdapter<String>(
        this, android.R.layout.simple_list_item_1, items);
        lv.setAdapter(adapter);

        Button button1main = (Button) findViewById(R.id.add);
        button1main.setOnClickListener(new OnClickListener()  {
            @Override
            public void onClick(View v)  {
            final Dialog additem = new Dialog(ShoppingList.this);
            additem.setContentView(R.layout.maindialog);
            additem.setTitle("Type your item");
            additem.setCancelable(true);
            et.setHint("Type the name of an item...");

        Button button = (Button) findViewById(R.id.cancel);
        button.setOnClickListener(new OnClickListener()  {
                @Override
                public void onClick(View v)  {
                    additem.dismiss();
                }
            });
            additem.show();

        Button ok = (Button) findViewById(R.id.ok);
        ok.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                items.add(et.getText().toString());
                adapter.notifyDataSetChanged();
                additem.dismiss();
                et.setText("");
        }

            }
        );
       }
        });
    }
}

Здесь мой main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="#FF9900"
    android:padding="10dp"
    android:orientation="vertical">
        <Button
            android:id="@+id/add"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Add item..."
            android:layout_gravity="center"/>
        <ListView android:id="@android:id/list"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:background="#FFCC00"/>  
</LinearLayout>

Здесь мой диалог xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:id="@+id/layout_root"
  android:orientation="vertical"
  android:background="#FFFFFF"
  android:minHeight="100dp"
  android:minWidth="300dp">
  <EditText
     android:id="@+id/edittext"
     android:layout_width="fill_parent"
     android:layout_height="wrap_content"/>
 <LinearLayout
    android:id="@+id/button_layout"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal"
    android:layout_gravity="bottom">
  <Button
    android:id="@+id/ok"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Ok"/>
  <Button
    android:id="@+id/cancel"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Cancel"/>
  </LinearLayout>   
</LinearLayout>