[SalesForce] Attempt to de-reference a null object

public with sharing class UserTriggerMethod {
public static void updateAppDelegation(Map<ID,User> mapNewUser)
{       
    // get a list of emails for the newly inserted users
    List<String> useremails = new List<String>();
    for (User u : mapNewUser.values()) 
    {
        if(u.ContactId!=null)
        {
            useremails.add(u.email);
        }
    }

    //query all Delegated contacts (and associated Application Delegations) at once
    List<Delegated_Contact__c> lstdelegatedContact = [SELECT Id, Email__c, 
                                                        (SELECT Contact__c from Application_Delegations__r) 
                                                        FROM Delegated_Contact__c 
                                                        WHERE Email__c in :useremails ];

    //create a lookup from email to Delegated Contact
    map<string, Delegated_Contact__c> mapemailToDelegatedContact= new map<String, Delegated_Contact__c>();
    for(Delegated_Contact__c delgatedContact : lstdelegatedContact) 
    {
        mapemailToDelegatedContact.put(delgatedContact.Email__c, delgatedContact);
    }

    List<Application_Delegation__c> lstAppDelegatedContoupdate;
    List<Delegated_Contact__c> lstDelegatedContacttodelete;

    for(User user : mapNewUser.values()) 
    {
        if(mapemailToDelegatedContact.containsKey(user.email)) 
        { 
            Delegated_Contact__c delegatedContact = mapemailToDelegatedContact.get(user.email);

            List<Application_Delegation__c> lstAppDelegatedContact = delegatedContact.Application_Delegations__r;              

            lstAppDelegatedContoupdate = new List<Application_Delegation__c>();
            lstDelegatedContacttodelete = new List<Delegated_Contact__c>();

            for(Application_Delegation__c AppDelegatedContact: lstAppDelegatedContact)
            {
                AppDelegatedContact.Contact__c = user.ContactId;
                lstAppDelegatedContoupdate.add(AppDelegatedContact);
            }

            lstDelegatedContacttodelete.add(delegatedContact);
        }
    }

    if(lstAppDelegatedContoupdate.size() > 0 && lstAppDelegatedContoupdate != null)
    {
        try
        {
            update lstAppDelegatedContoupdate;
        }
        catch(DMLException e)
        {
            system.debug(e.getMessage());
        }
    }

    if(!lstDelegatedContacttodelete.IsEmpty())
    {
        delete lstDelegatedContacttodelete;
    }   
}

}

Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger UserTrigger caused an unexpected exception, contact your administrator: UserTrigger: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object: Class.UserTriggerMethod.updateAppDelegation: line 52, column 1

i.e Line :-

if(lstAppDelegatedContoupdate.size() > 0 && lstAppDelegatedContoupdate != null)