I am trying to solve the Trailhead Platform Developer 1 (PD1) maintenance module I am facing the following error please help me to solve it. "The correct Safe Navigation logic was not found. Your Apex code should use the Safe Navigation operator to check for null values for Account Name. Please review line 18 of the original code and recheck your code for the Safe Navigation operator."..?

@RestResource(urlMapping='/apexSecurityRest')
global with sharing class ApexSecurityRest {
    @HttpGet
    global static Contact doGet() {
        Id recordId = RestContext.request.params.get('id');
        Contact result;
        if (recordId == null) {
           throw new FunctionalException('Id parameter is required');
        }
        if (Schema.SObjectType.Contact.isAccessible()
          && Schema.SObjectType.Contact.fields.Name.isAccessible()
          && Schema.SObjectType.Contact.fields.Top_Secret__c.isAccessible()
        ) {
          List<Contact> results = [SELECT id, Name, Title, Top_Secret__c, Account.Name FROM Contact WHERE Id = :recordId];
          if (!results.isEmpty()) {
             result = results[0];
             if (Schema.sObjectType.Contact.fields.Description.isUpdateable()){
                 result.Description = result.Account.Name;
                 }
             }
           } else {
             throw new SecurityException('You don\'t have access to all contact fields required to use this API');
           }
           return result;
      }
      public class FunctionalException extends Exception{}
      public class SecurityException extends Exception{}
}

 

https://developer.salesforce.com/trailhead/advanced_formulas/picklist_formulas
Has anyone managed to pass the challenge?
Gives error:
Challenge Not yet complete... here's what's wrong: 
The validation rule does not appear to be working correctly. Marking IsEscalated to true and Priority to Medium did not fire the validation rule.
Testing that same case myself (and some permuations) the Validation rule fires correctly I think...
Here is my validation formula
AND(
ISPICKVAL( Status , "Escalated"),
OR(
IsClosed,
IsClosedOnCreate,
NOT(ISPICKVAL( Priority , "High"))
)
)