I already create a table at the same access file but when I save information to that database it pop out NullReferenceException

Question

I'm doing a todo list vb.net, I have 2 button name Complete and Execute, when the user press Complete, the interface will let user to choose which task has been executed, and the task is from table name todolist and if the user press Execute on this task then it will do a copy to table name complete. But every time I run the program it appear NullreferenceException, but I already create the table.

I wonder is there my logic thinking or the system? Can anyone tell me where is my mistake?

Here is my Execute button

Private Sub btnexecute_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnexecute.Click
    If inc <> -1 Then
        Dim cb As New OleDb.OleDbCommandBuilder(daa)
        cb.QuotePrefix = "["
        cb.QuoteSuffix = "]"
        Dim dsNewRow As DataRow
        con.Open()
        dsNewRow = dss.Tables("complete").NewRow() `exception occur`

    dsNewRow.Item("Task") = txtitem.Text
    dsNewRow.Item("description") = txtdescript.Text
    dsNewRow.Item("Date of create") = dtptoday.Text
    dsNewRow.Item("Date of execute") = dtpcomplete.Text

    dss.Tables("complete").Rows.Add(dsNewRow) 
    MaxRows = MaxRows + 1

    da.Update(dss, "complete")
    con.Close()
    MsgBox("Task has been executed, will be remove here and put in!")

`enable and disable button
    btncomplete.Enabled = True
    btnSave.Enabled = False
    btnadd.Enabled = True
    btnUpdate.Enabled = True
    btnremove.Enabled = True

    inc = 0

`navigate records
    NavigateRecords()

End If
End Sub

Here is my complete button

  Private Sub btncomplete_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btncomplete.Click

    btnSave.Enabled = False
    btnadd.Enabled = False
    btnUpdate.Enabled = False
    btnremove.Enabled = False
    btnexecute.Enabled = True

`call back the record from todolist table
    NavigateRecords()

End Sub