polish #1586

core/src/main/java/com/taobao/arthas/core/command/express/ExpressFactory.java

@@ -24,6 +24,9 @@ public class ExpressFactory {
     }
 
     public static Express unpooledExpress(ClassLoader classloader) {
+        if (classloader == null) {
+            classloader = ClassLoader.getSystemClassLoader();
+        }
         return new OgnlExpress(new ClassLoaderClassResolver(classloader));
     }
 }