Best way to handle NULLs

What's the best way to deal with NULL values in VB.net? If one of the values I'm receiving from the database is NULL I get "ErrorSystem.InvalidCastException: Cast from type 'DBNULL' to type 'String' is not valid'

   Private Function LoadOrgansation() As Boolean()
        Try

            'Check the status of the connection
            CheckConnectionStatus()

            Dim cmd As SqlCommand = cnn.CreateCommand
            cmd.CommandType = CommandType.StoredProcedure
            cmd.CommandText = "procLoadOrganisation"

            'Input Parameter
            cmd.Parameters.Add(New SqlParameter("@OrganisationIDAuto", SqlDbType.Int))
            cmd.Parameters("@OrganisationIDAuto").Value = CInt(txt_ServiceProvider.Tag)

            Dim reader As SqlDataReader = cmd.ExecuteReader
            If reader.Read Then
                With reader
                    txt_OrganisationName.Text = CType(.GetValue(.GetOrdinal("ProviderName")), String)
                    txt_PostalAddress.Text = CType(.GetValue(.GetOrdinal("Address")), String)
                    txt_PostalCity.Text = CType(.GetValue(.GetOrdinal("City")), String)
                    txt_Phone.Text = CType(.GetValue(.GetOrdinal("Phone")), String)
                    txt_Email.Text = CType(.GetValue(.GetOrdinal("EMail")), String)
                    txt_ContactPerson.Text = CType(.GetValue(.GetOrdinal("ContactPerson")), String)
                    txt_OrganisationNotes.Text = CType(.GetValue(.GetOrdinal("Description")), String)
                End With
                reader.Close()
            End If

        Catch ex As Exception
            MessageBox.Show("ERROR" & ex.ToString)
        Finally
            cnn.Close()
        End Try
    End Function