breaking exception '_InternalLinkedHashMap<dynamic, dynamic>' is not a subtype of type 'Map<String, String>'

The constructor for Uri.https requires a Map with a runtime type of Map<String, String>. When you create stringParams without any type annotations, you are actually creating a Map<dynamic, dynamic>. The correct way to create this for Dart 2 is

Map<String, String> stringParams = {};
// or
var stringParams = <String, String>{};

The reason this used to work is that in Dart 1, even in strong mode, dynamic was fuzzy and acted like both Object and null - meaning a dynamic type was assignable to and from anything. In Dart 2, dynamic acts just like Object, except you can call methods or access properties on it without a downcast.

I used this

    if(json["key"]!= null){
       this.active_guests = json["key"].cast<String, int>();
     }