This is the right phase to fill in all the descriptors

core/src/main/java/hudson/ExtensionList.java

@@ -252,9 +252,12 @@ public class ExtensionList<T> extends AbstractList<T> {
             if (extensions==null)
                 return;     // not yet loaded. when we load it, we'll load everything visible by then, so no work needed
 
-            List<ExtensionComponent<T>> l = Lists.newArrayList(extensions);
-            l.addAll(load(delta));
-            extensions = sort(l);
+            Collection<ExtensionComponent<T>> found = load(delta);
+            if (!found.isEmpty()) {
+                List<ExtensionComponent<T>> l = Lists.newArrayList(extensions);
+                l.addAll(found);
+                extensions = sort(l);
+            }
         }
     }
 

core/src/main/java/hudson/PluginManager.java

@@ -344,6 +344,8 @@ public abstract class PluginManager extends AbstractModelObject {
             p.resolvePluginDependencies();
             strategy.load(p);
 
+            Jenkins.getInstance().refreshExtensions();
+
             p.getPlugin().postInitialize();
         } catch (Exception e) {
             failedPlugins.add(new FailedPlugin(p.getShortName(), e));
@@ -361,7 +363,6 @@ public abstract class PluginManager extends AbstractModelObject {
             }
         }.discoverTasks(r));
         new InitReactorRunner().run(r);
-        Jenkins.getInstance().refreshExtensions();
     }
 
     /**