fix(plc4j/opcua): Fix null pointer exception in OPC UA Subscription Handle.

The 'sampling' mode in case of OPC-UA means that no actual events are generated to the client.
This is a sort of 'suspended' subscription which will deliver its events once monitoring item is switched back to reporting mode.
In essence all UA subscriptions must be in reporting mode to generate data for client.

Closes #2024.

Signed-off-by: Łukasz Dywicki <[email protected]>

Author: splatch

plc4j/drivers/opcua/src/main/java/org/apache/plc4x/java/opcua/protocol/OpcuaSubscriptionHandle.java

@@ -107,10 +107,6 @@ public CompletableFuture<OpcuaSubscriptionHandle> onSubscribeCreateMonitoredItem
                 new QualifiedName(0, OpcuaProtocolLogic.NULL_STRING));
 
             MonitoringMode monitoringMode = MonitoringMode.monitoringModeReporting;
-            if (PlcSubscriptionType.CYCLIC == tagDefaultPlcSubscription.getPlcSubscriptionType()) {
-                monitoringMode = MonitoringMode.monitoringModeSampling;
-            }
-
             ExtensionObject eventFilter = OpcuaProtocolLogic.NULL_EXTENSION_OBJECT;
             if (tagDefaultPlcSubscription.getPlcSubscriptionType() == PlcSubscriptionType.EVENT) {
                 NodeId nodeId = new NodeId(new NodeIdFourByte((short) 0, OpcuaNodeIdServicesObjectType.BaseEventType.getValue()));
@@ -219,22 +215,25 @@ private void sendPublishRequest() {
                         outstandingAcknowledgements.add(new SubscriptionAcknowledgement(this.subscriptionId, availableSequenceNumber));
                     }
 
-                    for (ExtensionObject notificationMessage : responseMessage.getNotificationMessage().getNotificationData()) {
-                        ExtensionObjectDefinition notification = notificationMessage.getBody();
-                        if (notification instanceof DataChangeNotification) {
-                            logger.trace("Found a Data Change Notification");
-                            DataChangeNotification data = (DataChangeNotification) notification;
-                            if (!data.getMonitoredItems().isEmpty()) {
-                                onMonitoredValue(data.getMonitoredItems());
-                            }
-                        } else if (notification instanceof EventNotificationList) {
-                            logger.trace("Found a Event Notification");
-                            EventNotificationList data = (EventNotificationList) notification;
-                            if (!data.getEvents().isEmpty()) {
-                                onEventNotification(data.getEvents());
+                    NotificationMessage message = responseMessage.getNotificationMessage();
+                    if (message.getNotificationData() != null) {
+                        for (ExtensionObject notificationMessage : message.getNotificationData()) {
+                            ExtensionObjectDefinition notification = notificationMessage.getBody();
+                            if (notification instanceof DataChangeNotification) {
+                                logger.trace("Found a Data Change Notification");
+                                DataChangeNotification data = (DataChangeNotification) notification;
+                                if (!data.getMonitoredItems().isEmpty()) {
+                                    onMonitoredValue(data.getMonitoredItems());
+                                }
+                            } else if (notification instanceof EventNotificationList) {
+                                logger.trace("Found a Event Notification");
+                                EventNotificationList data = (EventNotificationList) notification;
+                                if (!data.getEvents().isEmpty()) {
+                                    onEventNotification(data.getEvents());
+                                }
+                            } else {
+                                logger.warn("Unsupported Notification type {}", notification.getClass().getName());
                             }
-                        } else {
-                            logger.warn("Unsupported Notification type {}", notification.getClass().getName());
                         }
                     }
                 }).whenComplete((result, error) -> {