Rejecting a promise throws "Uncaught (in promise)"

just like Amaya said, you must use catch to catch the error, otherwise it will become a UnhandledPromiseRejection, different promise vender have different implementations to handle such kind of UnhandledPromiseRejection, but in either way, it will just be thrown at some point, so zone.js will throw the error in the next tick.

And I see you have already used finally, if you use finally UnhandledPromiseRejection should not be thrown, but you need to use zone.js 0.8.26 to support Promise.finally, if the error still exists when you upgrade the zone.js, please tell me or fire an issue in zone.js repository, https://github.com/angular/zone.js/issues

You have to catch it to prevent the error

modal.promise.then(hideFn, hideFn).catch((res) => {});