strengthen shutdown

Author: marregui

hivemq-edge/src/main/java/com/hivemq/HiveMQEdgeMain.java

@@ -174,10 +174,12 @@ public void start(final @Nullable EmbeddedExtension embeddedExtension)
 
     public void stop() {
         stopGateway();
-        try {
-            Runtime.getRuntime().removeShutdownHook(shutdownThread);
-        } catch (final IllegalStateException ignored) {
-            //ignore
+        if (shutdownThread != null) {
+            try {
+                Runtime.getRuntime().removeShutdownHook(shutdownThread);
+            } catch (final IllegalStateException ignored) {
+                //ignore
+            }
         }
     }