check if getCallerThreadPoolExecutor is null or not #1265

Author: lling

marklogic-client-api/src/main/java/com/marklogic/client/dataservices/impl/ExecEndpointImpl.java

@@ -254,7 +254,7 @@ public Boolean call() throws InterruptedException{
                     submitTask(this);
                 }
                 else {
-                    if (aliveCallContextCount.decrementAndGet() == 0) {
+                    if (aliveCallContextCount.decrementAndGet() == 0 && getCallerThreadPoolExecutor() != null) {
                         getCallerThreadPoolExecutor().shutdown();
                     }
                 }