Shutter 2 Advanced (II): shutter air safety

from Flutter 2 Start ,Flutter Empty security is enabled by default in the configuration , Prevent collapse .

If sdk Greater than 2.12.0 , Empty safety check is enabled ：

environment:
sdk: ">=2.12.0 <3.0.0" 

If you want to close , Transfer to 2.12.0 Here's how it goes ：

 environment:
  sdk: ">=2.7.0 <3.0.0"

The created variables are empty and safe by default , If NULL is allowed , To add ? , Such as ：

 String? url;

In the constructor, the variable cannot be empty. You need to add  required , Such as ：

  CommonModel(
      {required this.url,
      required this.statusBarColor,
      required this.hideAppBar});