flutter AlertDialog去掉阴影

以下是AlertDialog的构造方法。

const AlertDialog({
    Key? key,
    this.title,
    this.titlePadding,
    this.titleTextStyle,
    this.content,
    this.contentPadding = const EdgeInsets.fromLTRB(24.0, 20.0, 24.0, 24.0),
    this.contentTextStyle,
    this.actions,
    this.actionsPadding = EdgeInsets.zero,
    this.actionsOverflowDirection,
    this.actionsOverflowButtonSpacing,
    this.buttonPadding,
    this.backgroundColor,
    this.elevation,
    this.semanticLabel,
    this.insetPadding = _defaultInsetPadding,
    this.clipBehavior = Clip.none,
    this.shape,
    this.scrollable = false,
  }) : assert(contentPadding != null),
       assert(clipBehavior != null),
       super(key: key);

title：标题
titlePadding：标题内边距
titleTextStyle：标题样式
content：内容，推荐用SingleChildScrollView包裹
contentPadding：EdgeInsets.fromLTRB(24.0, 20.0, 24.0, 24.0)，内容内边距
contentTextStyle：内容样式
actions：按钮
actionsPadding：EdgeInsets.zero，actions内边距
buttonPadding：按钮内边距
backgroundColor：背景色
elevation：阴影
shape：形状
scrollable：滑动

当我们将backgroundColor设置为透明时，会有个阴影，我们如何去掉呢？

我尝试将elevation设置为0，阴影依然在，当我设置为0.0时阴影就消失了。

阴影的定义如下：

  /// {@macro flutter.material.dialog.elevation}
  /// {@macro flutter.material.material.elevation}
  final double? elevation;