i use treepanel in dropdown list but in sometimes i get this error, i build treepanel nodes in button click (CodeBehind)

please help me

Server Error in '/' Application.
Unexpected state: Finished. Line 1, position 3.
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: Newtonsoft.Json.JsonReaderException: Unexpected state: Finished. Line 1, position 3.

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:

[JsonReaderException: Unexpected state: Finished. Line 1, position 3.]
   Newtonsoft.Json.JsonTextReader.Read() +487
   Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings) +226
   Ext.Net.JSON.Deserialize(String value, Type type, List`1 converters, IContractResolver resolver) +287
   Ext.Net.JSON.Deserialize(String value, Type type, IContractResolver resolver) +73
   Ext.Net.JSON.Deserialize(String value, IContractResolver resolver) +111
   Ext.Net.TreePanel.LoadPostData(String postDataKey, NameValueCollection postCollection) +1374
   Ext.Net.PanelBase.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) +79
   System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) +1068
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3406

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.33440