内嵌服务销毁逻辑优化；

xxl-job-core/src/main/java/com/xxl/job/core/executor/XxlJobExecutor.java

@@ -159,12 +159,14 @@ public class XxlJobExecutor  {
 
     private void stopEmbedServer() {
         // stop provider factory
+        if (embedServer != null) {
             try {
                 embedServer.stop();
             } catch (Exception e) {
                 logger.error(e.getMessage(), e);
             }
         }
+    }
 
 
     // ---------------------- job handler repository ----------------------