I'm binding a WebDataGrid on a OnSelectedIndexChanged event which works fine. However I'm using an UnboundCheckBoxField and if I select checkbox in the header, then deselect one or more rows, and change the selection from the dropdownlist, I get the above error. 

I am not concerned with keeping previously checked values so I just clear DataSource, dehydrate, and rebind.

protected void serviceTypeFamilyDropDown_OnSelectedIndexChanged(object sender, EventArgs e)

{
  using (DropDownList serviceTypeFamilyDropDown = sender as DropDownList)
  {
    servicesGrid.ClearDataSource();
    servicesGrid.DataSource = ExportableServiceVisits.GetServiceVisitsByServiceTypeFamily(
      serviceTypeFamilyDropDown.SelectedItem.Value.GetDoubleOrIntValue<int>(),
      serviceDatePicker.Date);
    servicesGrid.DataBind();
  }
}

Following suggestions from this forum I created a property on the page and upon OnSelectedIndexChanged I updated the data of the property then on servicesGrid_OnPreRender, I cleared the DataSource, set the DataSource to the property and rebound. However, the error was thrown before OnPreRender() was executed.

private IEnumerable<ExportableServiceVisits> ServiceVisits { get; set; }

protected void serviceTypeFamilyDropDown_OnSelectedIndexChanged(object sender, EventArgs e)
{
  using (DropDownList serviceTypeFamilyDropDown = sender as DropDownList)
  {
    ServiceVisits = ExportableServiceVisits.GetServiceVisitsByServiceTypeFamily(
    serviceTypeFamilyDropDown.SelectedItem.Value.GetDoubleOrIntValue<int>(),
    serviceDatePicker.Date);
  }
}

protected void servicesGrid_OnPreRender(object sender, EventArgs e)
{
  servicesGrid.ClearDataSource();
  servicesGrid.DataSource = ServiceVisits;
  servicesGrid.DataBind();
}

What's the problem?