To reproduce this, drop an empty file named foo.xml somewhere under the deploy dir (I dropped mine under an exploded ear, but I don't think it matters if it's under an ear). The file will fail to deploy, but a DeploymentInfo will still be created for it, e.g.:

[org.jboss.deployment.DeploymentInfo@ee0f6e74

{ url=file:/C:/Projects/rhq-trunk/dev-container/jbossas/server/default/deploy/rhq.ear/rhq-downloads/rhq-plugins/foo.xml }

deployer: null
status: null
state: INIT_WAITING_DEPLOYER
watch: file:/C:/Projects/rhq-trunk/dev-container/jbossas/server/default/deploy/rhq.ear/rhq-downloads/rhq-plugins/foo.xml
altDD: null
lastDeployed: 1222197704045
lastModified: 1222197704045
mbeans:
]

Now go the MainDeployer MBean in the jmx-console and try to invoke the listDeployedModules() operation. You should get the following exception:

javax.management.RuntimeMBeanException
org.jboss.mx.interceptor.ReflectedDispatcher.handleInvocationExceptions(ReflectedDispatcher.java:176)
org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:163)
org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
org.jboss.mx.interceptor.AbstractInterceptor.invoke(AbstractInterceptor.java:133)
org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
org.jboss.mx.interceptor.ModelMBeanOperationInterceptor.invoke(ModelMBeanOperationInterceptor.java:142)
org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:264)
org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
org.jboss.jmx.adaptor.control.Server.invokeOpByName(Server.java:258)
org.jboss.jmx.adaptor.control.Server.invokeOp(Server.java:223)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.invokeOp(HtmlAdaptorServlet.java:262)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.processRequest(HtmlAdaptorServlet.java:100)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.doPost(HtmlAdaptorServlet.java:82)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.rhq.helpers.rtfilter.filter.RtFilter.doFilter(RtFilter.java:113)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)

root cause

java.lang.NullPointerException
org.jboss.deployment.SerializableDeploymentInfo.<init>(SerializableDeploymentInfo.java:102)
org.jboss.deployment.MainDeployer.listDeployedModules(MainDeployer.java:261)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
org.jboss.mx.interceptor.AbstractInterceptor.invoke(AbstractInterceptor.java:133)
org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
org.jboss.mx.interceptor.ModelMBeanOperationInterceptor.invoke(ModelMBeanOperationInterceptor.java:142)
org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:264)
org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
org.jboss.jmx.adaptor.control.Server.invokeOpByName(Server.java:258)
org.jboss.jmx.adaptor.control.Server.invokeOp(Server.java:223)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.invokeOp(HtmlAdaptorServlet.java:262)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.processRequest(HtmlAdaptorServlet.java:100)
org.jboss.jmx.adaptor.html.HtmlAdaptorServlet.doPost(HtmlAdaptorServlet.java:82)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.rhq.helpers.rtfilter.filter.RtFilter.doFilter(RtFilter.java:113)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)

The NPE occurs on the following line in the constructor of SerializableDeploymentInfo:

this.deployer = info.deployer.getServiceName();

because info.deployer is null.

I think the fix is changing the line to:

this.deployer = (info.deployer != null) ? info.deployer.getServiceName() : null;

This is an important fix for JON, because the listDeployedModules() JMX operation is relied on for EAR/WAR discovery.