Add custom DropDown to RadFilter

[Chamara]

i'm trying to add a RadFilter to filter my RadGrid which has several coulmns.it works fine. now i need to add a dropdown only for a one column filter.i have a column named  Revision in my RadGrid.i'm trying the following but it doesn't work

   <telerik:RadFilter runat="server" ID="RadFilter1" 
                FilterContainerID="gridSearchL3" ShowApplyButton="true" Height="100px" >
                <FieldEditors>
                <custom:RadFilterDropDownEditor DataValueField="Revision" DataTextField="Revision" DataType="System.String" FieldName="Revision" />
       
                </FieldEditors>
                </telerik:RadFilter>

i'm getting the error " Object reference not set to an instance of an object"
for creating custom dropdown i've used following class

http://www.telerik.com/help/aspnet-ajax/filter-custom-editors.html 

[Tsvetina]

Hi Chamara,

Can you confirm that you have assigned the custom class in the FieldEditorCreating event?

protected void RadFilter1_FieldEditorCreating(object sender, RadFilterFieldEditorCreatingEventArgs e)
{
    if (e.EditorType == "RadFilterDropDownEditor")
    {
        e.Editor = new RadFilterDropDownEditor();
    }
}

If this is not the issue, please share your code, so we can look for any possible mistakes.

Greetings,
Tsvetina
the Telerik team