如何使用edittext从自定义对话框窗口获取输入

Question

如何使用EditText从自定义对话框窗口获取输入？我使用下面的代码得到一个空指针异常，editText为null，我尝试了这篇文章How to get data from a custom dialog box上的建议，但它没有用。这是我的代码：

package se315.journal;

import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.AdapterView;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;

public class MarkActivity extends AppCompatActivity
{
    ArrayList<Item> items;
    EditText editText;

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

    DBHelper dbHelper = new DBHelper(this);
    editText = new EditText(this);
    Mark mark = (Mark) getIntent().getSerializableExtra("Mark");
    Student student = dbHelper.getStudent(mark.getStudentRegister());
    Subject subject = dbHelper.getSubject(mark.getSubjectId());
    String title = student.getSurName().substring(0, 1) + ".\t" +student.getName() + ",\t" + subject.getName();
    getSupportActionBar().setTitle(title);

    items = dbHelper.getSubjectItems(subject);
    ListView listView = (ListView) findViewById(R.id.mark_lv);
    final ListAdapter listAdapter = new ListAdapter(this, R.layout.list_item, items);
    listView.setAdapter(listAdapter);

    listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener()
    {
        @Override
        public boolean onItemLongClick(AdapterView<?> adapterView, View view, int position, long l)
        {
            final Item item = items.get(position);

            AlertDialog.Builder builder = new AlertDialog.Builder(MarkActivity.this);
            LayoutInflater inflater = MarkActivity.this.getLayoutInflater();
            builder.setView(inflater.inflate(R.layout.mark_dialog, null));
            editText = (EditText) findViewById(R.id.mark_dialog_et);
            builder.setMessage("Дүнгээ оруулна уу").setTitle("Сонгосон шалгалт/даалгаварын дүнг өөрчлөх");

            builder.setPositiveButton("Өөрчлөх", new DialogInterface.OnClickListener()
            {
                public void onClick(DialogInterface dialog, int id)
                {
                    item.setMark(Integer.valueOf(editText.getText().toString()));
                }
            });
            builder.setNegativeButton("Болих", new DialogInterface.OnClickListener()
            {
                public void onClick(DialogInterface dialog, int id)
                {
                    // User cancelled the dialog
                }
            });
            AlertDialog dialog = builder.create();
            dialog.show();
            return false;
        }
    });
}

}

对话框窗口的xml文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="wrap_content"
android:layout_height="wrap_content">

<EditText
    android:id="@+id/mark_dialog_et"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="16dp"
    android:layout_marginLeft="4dp"
    android:layout_marginRight="4dp"
    android:layout_marginTop="4dp"
    android:hint="@string/mark_tv_mark"
    android:inputType="number" />