getProblemCode method
Returns a brief problem code that can be displayed when linking to it.
Returns null
when URL has no problem.
Implementation
String? getProblemCode() {
if (isInvalid) return UrlProblemCodes.invalid;
if (!isSecure) return UrlProblemCodes.insecure;
if (!exists) return UrlProblemCodes.missing;
return null;
}