Error when selecting custom filter field editor

[Kyle Smith]

I am adding my field editors in code behind in Page_Init on every post pack (per the documentation). Some of the field editors are custom types. When I select the field associated with a custom editor, I get the following error:

Server Error in '/' Application.

---

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 

[NullReferenceException: Object reference not set to an instance of an object.] Telerik.Web.UI.RadFilterDataFieldEditor.CreateEditorFrom(RadFilterDataFieldEditor baseEditor) +110 Telerik.Web.UI.RadFilterDataFieldEditorCollection.RetrieveEditorForFieldName(String fieldName) +68 Telerik.Web.UI.RadFilter.ChangeExpressionFieldName(RadFilterSingleExpressionItem item, String fieldName) +124 Telerik.Web.UI.RadFilterCommandEventArgs.ExecuteCommand(Object source) +1277 Telerik.Web.UI.RadFilter.OnBubbleEvent(Object source, EventArgs args) +219 System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +37 Telerik.Web.UI.RadFilterExpressionItem.OnBubbleEvent(Object source, EventArgs args) +120 Telerik.Web.UI.RadFilterExpressionItem.FireCommandEvent(String commandName, Object commandArgument) +88 Telerik.Web.UI.RadFilter.HandleClientCommand(String commandName, String commandArgs) +791 Telerik.Web.UI.RadFilter.RaisePostBackEvent(String eventArgument) +127 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +9642338 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724

---

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18045


My custom rad filter field editor types are created like the example in the documentation. While debugging, none of the overridden methods in my custom editor are ever called.

I am using the latest 2013 Q2 release for .NET 4.5