Remove black background on custom dialog

I want to remove the black background on custom dialog as shown in the picture. I'm sure the black background was from the dialog, not from app's background.

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AlertDialog code

public class MyAlertDialog extends AlertDialog { 
    public MyAlertDialog(Context context) 
    {  
        super(context); 
    }  

    public MyAlertDialog(Context context, int theme) 
    { super(context, theme); }
}

Activity code

public void showMyDialogOK(Context context, String s, DialogInterface.OnClickListener OkListener) {        
    MyAlertDialog myDialog = new MyAlertDialog(context, R.style.MyDialog2);        
    myDialog.setTitle(null); 
    myDialog.setMessage(s);        
    myDialog.setButton(DialogInterface.BUTTON_POSITIVE ,"Ok", OkListener);
    myDialog.show();    
}

Styles

<?xml version="1.0" encoding="utf-8"?>
    <resources>
        <style name="MyTheme" parent="@android:style/Theme.NoTitleBar.Fullscreen">
            <item name="android:alertDialogStyle">@style/AlertDialog</item>  
        </style>    

        <style name="MyTheme2" parent="@android:style/Theme.Black">
            <item name="android:alertDialogStyle">@style/AlertDialog</item>    
        </style> 

        <style name="AlertDialog">        
            <item name="android:fullDark">@null</item>
            <item name="android:fullBright">@null</item>
            <item name="android:topDark">@drawable/popup_top_dark</item>
            <item name="android:topBright">@null</item>
            <item name="android:centerBright">@null</item>
            <item name="android:centerDark">@drawable/popup_center_dark</item>
            <item name="android:centerMedium">@null</item>
            <item name="android:bottomDark">@null</item>
            <item name="android:bottomBright">@null</item>
            <item name="android:bottomMedium">@drawable/popup_bottom_medium</item>
        </style>

        <style name="MyDialog2" parent="@android:Theme.Dialog">        
            <item name="android:windowBackground">@null</item>    
            <item name="android:buttonStyle">@style/CustomButton</item>  
        </style>    

        <style name="CustomButton" parent="@android:style/Widget.Button">        
            <item name="android:background">@drawable/button_stateful</item>  
        </style>
</resources>

Image resources

popup_center_dark.9.png

popup_bottom_medium.9.png

popup_top_dark.9.png

public MyAlertDialog(
        Context context, 
        int theme
    ) extends AlertDialog { 

    super(context, theme);
    getWindow().setBackgroundDrawable(new ColorDrawable(android.graphics.Color.TRANSPARENT));
}