Oh sorry, completely forgot to update the thread. It worked actually and it was all my mistake or I overlooked the concept myself.
Like mentioned in native SQL a condition should be " ((@IsPaid IS NOT NULL AND [IsPaid] = @IsPaid)OR @IsPaid IS NULL "
and in code I was using: ((isPaid != null && t.IsPaid == isPaid) || t.IsPaid == null))
Error was in last condition after || where it says t.IsPaid it should had been ((isPaid != null && t.IsPaid == isPaid) || isPaid == null)) (i.e the supplied parameter).
Hope its clear now :)
