DatabaseItem class Null safety

A class representing a single item to store using DatabaseStorage

Schema specified in the constructor is represented on the object as a readonly map. The keys will represent the column names and the case insensitive string values represent the type of data to be stored in that column. Notes:

• id can be excluded, it will automatically be set to STRING PRIMARY KEY
• adjusting this map will adjust the table schema by adding columns not currently in the database
• columns removed from the item schema will remain in the database

class Dog extends DatabaseItem {
  final String id;
  final String name;
  final int age;

  Dog(this.id, this.name, this.age): super(id, columns: {
    "name": "TEXT",
    "age": "integer"
  });
...
}