您需要调用showDialog

showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: Text(title),
          content: Text(message),

        );
      });

如Figen Güngör所述，您需要调用showDialog。只是想澄清一下，它是一个Future（函数），而不是一个Widget，因此您的代码如下所示：

Future updateProfilePic(BuildContext context){
    return showDialog(
      context: context,
      builder : (BuildContext context){
        return SimpleDialog(
          title: const Text('Select any option'),
          children: <Widget>[
            SimpleDialogOption(
              onPressed: () {  profileImg = ImagePicker.pickImage(source: ImageSource.gallery)
                  .whenComplete(() {
                setState(() {});
              }); },
              child: const Text('open gallery'),
            ),
            SimpleDialogOption(
              onPressed: () {   profileImg = ImagePicker.pickImage(source: ImageSource.camera)
                  .whenComplete(() {
                setState(() {});
              }); },
              child: const Text('open camera'),
            ),
          ],
        );
      },
    );
  }