That bug has been found by using the cascading service of Java DMK but is JMX specific.
The second queryNames() call gives a NPE with the following scenario.
Cascaded MBS is in another JVM, connector is JMXMP.

Test scenario:

cascadingService.start()
connector.createMBean(totoClass, totoName)
cascadingMBS.queryNames(null, null)
connector.unregisterMBean(totoName)
cascadingMBS.queryNames(null, null)

The stack:

java.lang.NullPointerException
    at javax.management.ObjectInstance.hashCode(ObjectInstance.java:99)
    at java.util.HashMap.hash(HashMap.java:261)
    at java.util.HashMap.put(HashMap.java:379)
    at java.util.HashSet.add(HashSet.java:192)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.filterListOfObjects(DefaultMBeanServerInterceptor.java:1501)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.queryMBeansImpl(DefaultMBeanServerInterceptor.java:490)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.queryNames(DefaultMBeanServerInterceptor.java:533)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.queryNames(JmxMBeanServer.java:600)
    at javasoft.sqe.jmx.test.cascading.CascadingTest.run(CascadingTest.java:69)
    at javasoft.sqe.jmx.share.TestStarter.<init>(TestStarter.java:40)
    at javasoft.sqe.jmx.share.TestStarter.main(TestStarter.java:98)