createDefaultValue method Null safety
Called by the RestorationMixin if no restoration data is available to restore the value of the property from to obtain the default value for the property.
The method returns the default value that the property should wrap if no restoration data is available. After this is called, initWithValue will be called with this method's return value.
The method may be called multiple times throughout the life of the RestorableProperty. Whenever new restoration data has been provided to the RestorationMixin the property is registered to, either this method or fromPrimitives is called before initWithValue is invoked.
@override DateTime createDefaultValue() => _defaultValue;