When ever you are working with grids, you need to make sure when you do
the findcontrol method that the control actually exists on that row of
the grid.

What trips up a lot of people when working with the OnItemEvents is the
header row and the footer row are run through here as well and as such
do not contain the controls you are looking for.

Dim drop1 As DropDownList = e.Item.FindControl("myDDL")
If not isNothing(drop1) then

Dim myDataSet As DataSet

Dim myDataTable As DataTable

myDataSet = CType(Session("data"), DataSet)

myDataTable = myDataSet.Tables("Tabel")

Dim myDataView As DataView

myDataView = New DataView(myDataTable)

drop1.DataSource = myDataView

drop1.DataMember = "Tabel"

drop1.DataTextField = "joke"

drop1.DataValueField = "aantal"

drop1.DataBind()

End if