I don't think that is a LLBL exception. However,
dashboardProduct.ProductItem[0].ProductItemWarehouse[0].ProductItemId = dashboardProduct.ProductItem[0].ProductItemId;
As I see, dashboardProduct.ProductItem[0].ProductItemId should be null at this point, as never has assigned at your code above.
dashboardProduct.ProductItem[0].ProductItemWarehouse[0].WarehouseId = productItemWarehouseIdWarehouseId; <<Gets Index out of Range Exc.
What's the value of productItemWarehouseIdWarehouseId variable at this point? Debug and watch what's going wrong.
Could you post the Exception's StackTrace and complete Exception Message.
David