made the test symmetric

core/src/main/java/hudson/ExtensionComponent.java

@@ -76,7 +76,7 @@ public class ExtensionComponent<T> implements Comparable<ExtensionComponent<T>>
         if (a<b)    return 1;
 
         // make the order bit more deterministic among extensions of the same ordinal
-        if (this.instance instanceof Descriptor) {
+        if (this.instance instanceof Descriptor && that.instance instanceof Descriptor) {
             return Util.fixNull(((Descriptor)this.instance).getDisplayName()).compareTo(Util.fixNull(((Descriptor)that.instance).getDisplayName()));
         } else {
             return this.instance.getClass().getName().compareTo(that.instance.getClass().getName());