Android自定义AlertDialog绘制不正确

我正在尝试自定义AlertDialog的外观。问题是我不知道如何消除出现在底部第一个TextView和EditText上方（灰色按钮区域上方）的黑线。

这是我的XML。我想念什么？

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:background="@color/butGreyBack"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">

      <TextView
        android:id="@+id/tv_title"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_gravity="center"
        android:layout_weight="30"
        android:text=""
        android:background="#00000000"
        android:textColor="@android:color/white"
        android:textSize="22sp"        
        android:layout_margin="0dp"     
        android:gravity="center" />

      <TextView
        android:id="@+id/tv_prompt"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_gravity="center"
        android:layout_weight="30"
        android:text=""
        android:background="#00000000"
        android:textColor="@android:color/white"
        android:textSize="18sp"        
        android:layout_margin="0dp"     
        android:gravity="center" />

    <EditText
        android:id="@+id/edit_text"
        android:inputType="textCapSentences|textAutoCorrect"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="4dp"
        android:layout_marginBottom="4dp"/>
</LinearLayout>

这也是代码-这可能就是问题所在：

  View alertView;

  AlertDialog.Builder builder = new AlertDialog.Builder( mContext );

  LayoutInflater inflater = LayoutInflater.from(getApplicationContext());
  alertView = inflater.inflate(R.layout.alert_dialog_text, null);

  builder.setView( alertView );

  AlertDialog alertDialog = builder.create();
  alertDialog.setView( alertView, 0,0,0,0 );

  final EditText input = (EditText) alertView.findViewById( R.id.edit_text );
  TextView tvTitle  = (TextView) alertView.findViewById( R.id.tv_title );
  TextView tvPrompt = (TextView) alertView.findViewById( R.id.tv_prompt );

  tvTitle.setText( "Rename Playlist" );
  tvPrompt.setText( "Enter new Playlist name." );

  // Set up the buttons
  builder.setPositiveButton( "OK", new DialogInterface.OnClickListener()
  {
    @Override
    public void onClick( DialogInterface dialog, int which )
    {
      mNewName = input.getText().toString();
    }
  } );

  builder.setNegativeButton( "Cancel", new DialogInterface.OnClickListener()
  {
    @Override
    public void onClick( DialogInterface dialog, int which )
    {
      mNewName = "";
      dialog.cancel();
    }
  } );

  builder.show();