Fix NPE in the UIMA AS client

The following NPE is thrown in the UIMA AS client:

WARNING:
java.lang.NullPointerException
at org.apache.uima.adapter.jms.client.BaseUIMAAsynchronousEngineCommon_impl.handleMetadataReply(BaseUIMAAsynch
ronousEngineCommon_impl.java:999)
at org.apache.uima.adapter.jms.client.BaseUIMAAsynchronousEngineCommon_impl.onMessage(BaseUIMAAsynchronousEngi
neCommon_impl.java:1678)
at org.apache.activemq.ActiveMQMessageConsumer.dispatch(ActiveMQMessageConsumer.java:1021)
at org.apache.activemq.ActiveMQSessionExecutor.dispatch(ActiveMQSessionExecutor.java:122)
at org.apache.activemq.ActiveMQSessionExecutor.iterate(ActiveMQSessionExecutor.java:192)
at org.apache.activemq.thread.PooledTaskRunner.runTask(PooledTaskRunner.java:122)
at org.apache.activemq.thread.PooledTaskRunner$1.run(PooledTaskRunner.java:43)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)

it seems that the application has called stop() method before a getMeta reply came back from a service. When stopping, the UIMA AS client invalidates some objects which leads to the NPE. Modify client code to check the state of the client while handling replies.