Hello All,
I have written a Trigger on OrderItems (before Insert) and now writing a test class for the same. 
I am getting an error as:

System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, Pricebook Not Set on Order: []

My Trigger:

trigger UnitPrice on OrderItem (before insert) {

    for (OrderItem orderItem: Trigger.new) {
    if(orderItem.UnitPrice == NULL){
            orderItem.UnitPrice= 0; // Dummy Value
            }
    }

}

Test Class:

@isTest(SeeAllData=true)
public class testUnitPrice {
    
    public static testMethod void testorderproduct(){

    // Insert Account

    Account a = new Account();
    a.Name = 'Test Account';
    insert a;

    // Insert Product
    Product2 p = new Product2();
    p.Name = ' Test Product ';
    p.Description='Test Product Entry 1';
    p.productCode = 'ABC';
    p.isActive = true;
    insert p;
    

    Pricebook2  standardPb = [select id, name, isActive from Pricebook2 where IsStandard = true limit 1];
    
    // Insert PricebookEntry

    PricebookEntry standardPrice = new PricebookEntry();
    standardPrice.Pricebook2Id = standardPb.Id;
    standardPrice.Product2Id = p.Id;
    standardPrice.UnitPrice = 1;
    standardPrice.IsActive = true;
    standardPrice.UseStandardPrice = false;
    insert standardPrice ;
    
    // Insert Order
    
    Order o = new Order();
    o.Name = 'Test Order ';
    o.Status = 'Draft';
    o.EffectiveDate = system.today();
    o.EndDate = system.today() + 4;
    o.AccountId = a.id;
    o.Pricebook2Id =  standardPb.Id ;
    insert o;
    
    // Insert Order Item

    OrderItem i = new OrderItem();
    i.OrderId = o.id;
    i.Quantity = 5;
    i.UnitPrice = 0;
    i.Product2id = p.id;
    i.PricebookEntryId=standardPrice.Id;
    insert i;

	}
}

I am very new to programming. Any help would be appreciated.

Thank you!
Maitreyee Dingare​​​​​​​​​​​​​​