I'm having basically the same problem as described here, with the same error message:

System.NullReferenceException: Object reference not set to an instance 
of an object.
   at umbraco.developer.assemblyBrowser.Page_Load(Object sender, 
EventArgs e)

I'm even more baffled, because all I've done is add another control to an existing assembly. After I update the assembly, the old controls/macros work fine, but when I try to add a new one, I always get this error. Even when the new control is a duplicate of an existing one that works, and the the assembly and type type names are spelled and set up the same as the old ones. It seems like Umbraco is just flaking out. Any ideas on how I can get this to magically start working like it did for the guy who started the thread I linked to? In the meantime I'll see if I can do what I need through an .ascx .