Cast exception in pwa registration

[SandraK82]

Unhandled Promise Rejection: Error: java.lang.ClassCastException in
protected void setupRegistration() {
if (isServiceWorkerSupported()) {
Navigator.serviceWorker.register(getResource(), getOption()).then(object -> {
logger.info("Service worker has been successfully registered");
registration = (ServiceWorkerRegistration) object;

(last line, File is ServiceWorkerManager)

Client is Safari 13.04, initial App runs in Superdevmode
not happening in 79.0.3945.117 from same superdevmode run

in Safari Debugger it is a:
object_0_0_g$: ServiceWorkerRegistration {installing: null, waiting: null, active: ServiceWorker, scope: "http://127.0.0.1:8888/", updateViaCache: "imports", …}

[SandraK82]

More clarifications:
I tested with current Safari on macOS Catalina and iOS. Both show class cast Exception.
Current Chrome works well on MacOS but not on iOS (I guess because of WebKit)