• Account recordtype is payment and requirement is like whenever this Payment recordtype account is not having any parent account then this Invoice_Number__c ParentId will be blank too.
• If this Payment recordtype account have parent record then this should be map into this Invoice_Number__c parentId.

I am trying to implement like this and  It is failing and getting null exception.

if(accRec.ParentId != null ){
inv.Invoice_Number__r.parentId = accRec.ParentId;
}

• Account recordtype is payment and requirement is like whenever this Payment recordtype account is not having any parent account then this Invoice_Number__c ParentId will be blank too.
• If this Payment recordtype account have parent record then this should be map into this Invoice_Number__c parentId.

I am trying to implement like this and  It is failing and getting null exception.

if(accRec.ParentId != null ){
inv.Invoice_Number__r.parentId = accRec.ParentId;
}