What is wrong with this code

[Mirnes]

Hello,

For last few days I'm trying everything to solve problem with this code but I still can't find any solution. Please, if you know what could it be, help.

There is code:

Dim ArtikalId, StopaId As Integer
Dim DefCijena, DefStopa As Double
If Me.NarudzbeKupacaStavkeDgr.Columns(e.ColumnIndex).Name = "Artikal" Then

            ArtikalId = Me.NarudzbeKupacaStavkeDgr.Rows(e.RowIndex).Cells("Artikal").Value
            Dim ArtikliRedak As DataRow = Me.NarudzbeKupacaDataSet.Tables("ListaArtikli").Rows.Find(ArtikalId)
            If ArtikliRedak IsNot Nothing Then
                DefCijena = ArtikliRedak("ArtikalCijena")
                NarudzbeKupacaStavkeDgr.Rows(e.RowIndex).Cells("Cijena").Value = DefCijena
                StopaId = ArtikliRedak("ArtikalStopa")                
            End If

            Dim StopeRedak As DataRow = Me.NarudzbeKupacaDataSet.Tables("SifarnikArtikli_StopePoreza").Rows.Find(StopaId)
            If StopeRedak IsNot Nothing Then
                DefStopa = StopeRedak("StopaStopa") 
                NarudzbeKupacaStavkeDgr.Rows(e.RowIndex).Cells("PdvStopa").Value = DefCijena
            End If

        End If

I got value of ArtikalId as well as DefCijena and StopaId but I don't get value of DefStopa. I checked names and values, there should be value of DefStopa.

When I put second part of this code, which should provide me value of DefStopa in try..catch like this:

Try
                Dim StopeRedak As DataRow = Me.NarudzbeKupacaDataSet.Tables("SifarnikArtikli_StopePoreza").Rows.Find(StopaId)
                DefStopa = StopeRedak("StopaStopa")
                NarudzbeKupacaStavkeDgr.Rows(e.RowIndex).Cells("PdvStopa").Value = DefStopa 
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try

I got message: "Object reference not set to an instance of an object".

Thanks in advance.