I have disabled breaking on System.NullReferenceException in Visual Studio 2012 Express for Web when that exception occured in debug mode. How to enable it now? Debug/Exceptions menu item is missing. I searched for System.NullReferenceException keys in registry, and value of State was always 16418 - same as other exceptions.
I believe (but can't verify right now) that those options are saved in the solution's suo (solution user options) file which is in the same directory as the solution (sln) file.
You can try to move the file to a different directory and see if your exception comes back. You'll loose any other customization you've done thought (start up project, breakpoints, watches, etc.)
Or you may try creating an extension and adding this functionality back to express :-)