Clearly HttpContext.Current is not null only if you access it in a thread that handles incoming requests. That’s why it works “when i use this code in another class of a page”.
It won’t work in the scheduling related class because relevant code is not executed on a valid thread, but a background thread, which has no HTTP context associated with.
Overall, don’t use Application["Setting"] to store global stuffs, as they are not global as you discovered.
If you need to pass certain information down to business logic layer, pass as arguments to the related methods. Don’t let your business logic layer access things like HttpContext or Application["Settings"], as that violates the principles of isolation and decoupling.
Update: Due to the introduction of async/await it is more often that such issues happen, so you might consider the following tip,
In general, you should only call HttpContext.Current in only a few scenarios (within an HTTP module for example). In all other cases, you should use
Page.Contexthttps://docs.microsoft.com/en-us/dotnet/api/system.web.ui.page.context?view=netframework-4.7.2Controller.HttpContexthttps://docs.microsoft.com/en-us/dotnet/api/system.web.mvc.controller.httpcontext?view=aspnet-mvc-5.2
instead of HttpContext.Current.