GH-1618: Fix ResourcesChange Event in SimpleInstanceEnabler

Author: sbernard31

leshan-client-core/src/main/java/org/eclipse/leshan/client/resource/SimpleInstanceEnabler.java

@@ -125,7 +125,7 @@ public WriteResponse write(LwM2mServer server, boolean replace, int resourceid,
             });
 
             newValue.getInstances().forEach((newInstanceId, newInstance) -> {
-                LwM2mResourceInstance previousInstance = newValue.getInstances().get(newInstanceId);
+                LwM2mResourceInstance previousInstance = previousValue.getInstances().get(newInstanceId);
                 if (previousInstance == null) {
                     // addition
                     changedResources.add(getResourceInstancePath(resourceid, newInstanceId));