Don't try to guess extension directory too soon.

see #197

admin/__init__.py

@@ -42,7 +42,7 @@ def enabled_applications():
     """
     result = [("admin", "admin"), ("userprefs", "userprefs")]
     try:
-        exts = list(Extension.objects.filter(enabled=True))
+        exts = Extension.objects.filter(enabled=True)
     except DatabaseError:
         exts = []
     result += [(ext.name, ext.name) for ext in exts]

admin/models.py

@@ -810,7 +810,8 @@ class Extension(models.Model):
         super(Extension, self).__init__(*args, **kwargs)
         if self.name:
             self.instance = exts_pool.get_extension(self.name)
-            self.__get_ext_dir()
+            if self.instance:
+                self.__get_ext_dir()
 
     def __get_ext_dir(self):
         modname = self.instance.__module__