How to solve below error from Ajax control tool kit?

User Agent: InternetExplorer/11.0
Error message: Exception of type 'System.Web.HttpUnhandledException' was thrown.
Error details:
System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.NullReferenceException: Object reference not set to an instance of an object. at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) at AjaxControlToolkit.ToolkitScriptManager.GetScriptCombineAttributes(Assembly assembly) at AjaxControlToolkit.ToolkitScriptManager.IsScriptCombinable(ScriptEntry scriptEntry) at AjaxControlToolkit.ToolkitScriptManager.OnResolveScriptReference(ScriptReferenceEventArgs e) at System.Web.UI.ScriptManager.RegisterScripts() at System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e) at System.Web.UI.Page.OnPreRenderComplete(EventArgs e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.brightableweb_new_dashboard_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\345b32c1\e002ef98\App_Web_1mnymemt.3.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)