This problem occurs in my codebehind file, not in the ascx file, so the code you suggest will not work. I am not directly calling GetCurrent() in my code - I am calling AddAjaxSetting() which internally calls GetCurrent(). If I call GetCurrent() myself as a test while debugging, it returns null.
Here is the code I use to call AddAjaxSetting:
ajax_manager.AjaxSettings.AddAjaxSetting(link_type_list, category_list, loading_panel);
When I call this, the ajax_manager variable is *not* null - it is correctly pointing to the ajax manager control on the page. Same for the controls passed as parameters - none of them are null.