Merge pull request #15216 from smatvienko-tb/fix/lwm2m-fw-update-flaky-test

Fix flaky Ota5LwM2MIntegrationTest.testFirmwareUpdateByObject5_Ok

application/src/test/java/org/thingsboard/server/transport/lwm2m/client/FwLwM2MDevice.java

@@ -171,7 +171,14 @@ public class FwLwM2MDevice extends BaseInstanceEnabler implements Destroyable {
 
                 if (this.leshanClient != null) {
                     log.info("Stop/reboot LwM2M client {}", this.leshanClient.getEndpoint(identity));
-                    this.leshanClient.stop(false);
+                    try {
+                        this.leshanClient.stop(false);
+                    } catch (Exception stopEx) {
+                        // Leshan may throw NPE during CoAP observe-relation cleanup when the server
+                        // reference is null (race condition in NotificationDataStore.toKey()).
+                        // The client is still considered stopped at this point — proceed with restart.
+                        log.warn("Exception during LwM2M client stop, proceeding with restart: {}", stopEx.getMessage());
+                    }
 
                     log.info("Start after update fw LwM2M client {}", this.leshanClient.getEndpoint(identity));
                     this.leshanClient.start();