Commented Issue: System.NullReferenceException: Object reference not set to an instance of an object. [26752]

From time to time, my clients gets this error.

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)
Comments: Attached a Visual Studio 2012 project that will reproduce this error using IIS Express and the September 2012 release for .NET 3.5. 1. Press F5 to start the site (ToolkitScriptManager and thread synchronization) 2. Right click the 'repro' project, 'Debug' > 'Start new instance' (10 simultaneous requests) 3. Press Enter to choose the default IIS Express test url 4. Watch up to 10 attempts to trigger the error, which is reported as error code 555. 5. Beware: IIS Express may hit 100% CPU usage Reproduced on Windows 8 Enterprise x64, i7-3770 3.4GHz (4 cores, 8 logical processors) with 16GB RAM. Some of the delays to allow time for various steps to complete are a little low for slower machines. Would appreciate a fix for this highest-voted, 2.5 year-old issue.