Hello, I need some help with my program for school.
Even my teacher can't fix it so I ask it here.
ooh and sorry for the language it's in Dutch...

First of all, when I click the button "show" (Tonen) after starting up the program fails. with this error (code line is in red):
Fill: SelectCommand.Connection property has not been initialized.

But when I do something else first it will actually work.

 Sub subMaakConnectie(ByVal strSQL As String)
        '********************************
        '* Maak connectie en vul dataset.
        '********************************
        ' Zoek pad naar database
        Dim strDBPad As String = fntPath()
        Dim da As New OleDbDataAdapter(strSQL, cnn)
        Dim ds As New DataSet()

        subOpenConnection()

        If strSQL = "" Then
            MessageBox.Show("Selecteer eerste een criteria om te zoeken", "ERROR")
        Else
            If TabControl1.SelectedTab Is tcKlanten Then
                subOpenConnection()
                da.Fill(ds, "SELECT * FROM tblKlanten")
                da.Dispose()
                cnn.Close()

                ' ...  en vul datagrid
                With dgKlanten
                    .AutoGenerateColumns = True
                    .DataSource = ds
                    .DataMember = "SELECT * FROM tblKlanten"
                End With...........

This is the code behind the button:

 Private Sub btnKlantTonen_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnKlantTonen.Click
        Dim strSQL As String = ""
        strSQL = "SELECT * FROM tblKlanten ORDER BY fldKlantNaam"
        subMaakConnectie(strSQL)
    End Sub

The second problem has something to do with making a combobox that uses the value of another combobox to make the SQL line.
Making the line is done, and for what i can see it should work but is doesn't.
This error occures at he red marked line:
No value given for one or more required parameters.

 Private Sub cmbBestelProduct_SelectedIndexChanged_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbBestelProduct.SelectedIndexChanged
        Dim strCommand As String
        Dim strField As String
        strCbSelect = "KlantVoornaam"
        strCommand = "'SELECT DISTINCT fldProductPrijs FROM tblProducten WHERE fldproductNaam=" & CStr(cmbBestelProduct.SelectedItem) & "'"
        'MessageBox.Show(strCommand)
        strCommand = strCommand.Substring(1, (strCommand.Length - 2))
        'MessageBox.Show(strCommand)
        strField = "fldProductenPrijs"
        fntZoeken(strCommand, strField)
    End Sub

the weird thing wit the substring above is my inventive way to make the proper command :)

 Function fntZoeken(ByVal strCommand As String, ByVal strField As String)
        subOpenConnection()

        ' 2. Maak het commando
        Dim cmd As OleDbCommand
        Dim strCmd As String = strCommand

        cmd = New OleDbCommand(strCmd, cnn)

        ' 3. Haal gegevens op
        Dim res As New List(Of String)
        Dim strItem As String
        Dim rd As OleDbDataReader
        'MessageBox.Show(strCmd)
        'MessageBox.Show(strField)


        rd = cmd.ExecuteReader(CommandBehavior.CloseConnection)

If rd.HasRows Then
                            Do While rd.Read
                                strItem = CStr(rd(strField))
                                res.Add(strItem)
                                cmbBestelKlantVoornaam.Items.Add(strItem)
                            Loop
                        End If
                        cmbBestelKlantVoornaam.DroppedDown = True

Many thanks in advance.