how do solve this error: Value cannot be null. Parameter name: No target control is set for the CascadingDropDown extender.

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ASP.NET

ASMX

Value cannot be null.
Parameter name: No target control is set for the CascadingDropDown extender.

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.ArgumentNullException: Value cannot be null.
Parameter name: No target control is set for the CascadingDropDown extender.

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:


[ArgumentNullException: Value cannot be null.
Parameter name: No target control is set for the CascadingDropDown extender.]
AjaxControlToolkit.CascadingDropDown.CascadingDropDown_ClientStateValuesLoaded(Object sender, EventArgs e) +406
AjaxControlToolkit.ExtenderControlBase.LoadClientStateValues() +187
AjaxControlToolkit.ExtenderControlBase.Page_PreLoad(Object sender, EventArgs e) +5
System.EventHandler.Invoke(Object sender, EventArgs e) +0
System.Web.UI.Page.OnPreLoad(EventArgs e) +121
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +699

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34249