Getting as error in the challenge section of Build a Conference Management App - Create a Lightning Component with Server-Side Apex Controller . The error says "There was an unexpected error in your org which is preventing this assessment check from completing: System.QueryException: List has no rows for assignment to SObject". 
This is what the server - side controller looks like: 
public class AttachmentController {
    @AuraEnabled
    public static void updatePicturePath(String recId){
        //In Lightning Experience, Attachments are stored in ContentDocuments
        ContentDocumentLink docLink = [ SELECT ContentDocumentId
                               FROM ContentDocumentLink
                               WHERE LinkedEntityId = :recId order by Id desc Limit 1];
        //ContentVersion Id uniquely identifies the attachment
        ContentVersion ver = [SELECT Id FROM ContentVersion Where ContentDocumentId = :docLink.ContentDocumentId];
        //Update the Picture_Path field with the url of the image
        Speaker__c speaker = [SELECT Id FROM Speaker__c WHERE Id = :recId];
        speaker.Picture_Path__c = '/sfc/servlet.shepherd/version/download/'+ ver.Id;
        upsert speaker;
    }
}