The argument type 'String?' can't be assigned to the parameter type 'String'.

Change the return type of statusCode to String and provide a default value.

String get statusCode => statusCodes[this] ?? '';

When accessing a map, there is a chance that you will get a null return value if the key does not exist in the map. Simply providing a default value will allow this code to compile. That default value should never be used unless you add something to the enum without adding a value to the map as well.

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