trigger SetContactOnOrderItem on Line_item__c (before update, before insert) {
    Set<Integer> lineitemnumbers = new Set<Integer>();

    for (Line_item__c collectNumFromLineitem : Trigger.new) {
        lineitemnumbers.add(collectNumFromLineitem.Name);
    }

    List<Opportunity> oppList = [SELECT id, Related_Sponsorship_Offer__c
 FROM Opportunity WHERE Related_Sponsorship_Offer__c IN :lineitemnumbers];

    Map<Integer, Opportunity> lineitemNumToOppMap = new Map<Integer, Contact>();

    for (Opportunity c : oppList) {
        lineitemNumToOppMap.put(c.Related_Sponsorship_Offer__c, c);
    }

    for (Line_item__c o : Trigger.new) {
        if (o.Name != null) {
            o.Opportunity__c = lineitemNumToOppMap.get(o.Name).id;
        }
        else {
            o.Opportunity__c = null;
        }
    }
}


I have a custom object named: Line_item__c and a standard object called opportunity. The custom object has a lookup field for an opportunity. The opportunity page also has a lookup field for the custom object. When a opportunity look up field is populated on the custom object, it should autopopulate the related Line item on the look up field on the opportunity. I tried this trigger but I get the following error. I am new the apex and any help would be appreciated.

Thank you