about combo box column of datagridview in vb.net

Question

i have combobox column and on selection of the combobox data i want the text field column of the datagrid view to get the text based on the selection of the data in the combobox column

the text in the textbox will come from the database based on data selected in combobox

i have done the bit its not working it is giving "Object reference not set to an instance of an object".

code is

can any one help me

Public cb As ComboBox

Private Sub dgv_assign_dboy_EditingControlShowing(ByVal sender As Object, _
                   ByVal e As System.Windows.Forms.DataGridViewEditingControlShowingEventArgs) _
                   Handles dgv_assign_dboy.EditingControlShowing
    ' Attempt to cast the EditingControl to a ComboBox.
    cb = TryCast(e.Control, ComboBox)
    If cb IsNot Nothing Then
        RemoveHandler cb.SelectedIndexChanged, AddressOf DGVComboIndexChanged
        AddHandler cb.SelectedIndexChanged, AddressOf DGVComboIndexChanged
    End If
End Sub

Private Sub DGVComboIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    'using the value selected in the cbox to find and insert the right value in the textbox

    strselect = "select fname from dboy where d_id='" & Trim(cb.SelectedValue.ToString) & "'"
    rstt = sqlconnect.readquery(strselect)
    If rstt.HasRows = True Then
        While rstt.Read
            dgv_assign_dboy.Item(0, dgv_assign_dboy.CurrentCell.RowIndex).Value = rstt(0).ToString
        End While
    End If