【错误记录】Flutter 编译报错 ( The parameter ‘‘ can‘t have a value of ‘null‘ because of its type, but the im )

The parameter 'icon' can't have a value of 'null' because of its type, 
but the implicit default value is 'null'. 
(Documentation)  Try adding either an explicit non-'null' default value or the 'required' modifier.

class CommonModel {
  final String? icon;
  final String? title;
  final String? url;
  final String? statusBarColor;
  final bool? hideAppBar;
  CommonModel({this.icon, this.title, this.url, this.statusBarColor, this.hideAppBar});
}