Review the CartProvider.Delete method

Description

Partner gets an exception which possibly seems to happen when CartProvider.Delete tries to delete a cart that is already deleted.

System.NullReferenceException: Object reference not set to an instance of an object.
at EPiServer.Commerce.Order.Internal.CartProvider.Delete(OrderReference orderLink)
at EPiServer.Commerce.Order.DefaultOrderRepository.Delete(OrderReference orderLink)
at EPiServer.Commerce.Order.Internal.CartMigrator.MigrateCarts(Guid sourceCustomerId, Guid destinationCustomerId)
at EPiServer.Commerce.Order.ProfileMigrator.MigrateCarts(Guid anonymousId)
at EPiServer.Business.Commerce.HttpModules.ProfileModule.Profile_MigrateAnonymous(Object sender, ProfileMigrateEventArgs pe)
at System.Web.Profile.ProfileModule.OnEnter(Object source, EventArgs eventArgs)
at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

They think that it might happen when two requests initiating cart migration at the same time. They think we should implement a null check on 

public void Delete(OrderReference orderLink)
{
Cart cart = CartProvider.EnsureCartType(this.Load(orderLink.OrderGroupId));
cart.Delete();
cart.AcceptChanges();
}