@Dorothy_lee - you have to add another and condition on the dtfiletred itself to control the null datarows…
update:
DtFiltered = (From r In Dtout.AsEnumerable
Where r("Feedback").toString.Contains("FALSE") and (not r("Feeback") is Nothing orelse
Not String.IsNullOrEmpty(r("Feeback").ToString))
Select r).CopyToDataTable