There are two issues in the previous implementation:
1. `_populateDevices` will return the devices from `deviceNotifier` if it had been initialized, assuming that once it's initialized, it has been properly populated. That assumption is not true because calling getters like `onAdded` would initialize `deviceNotifier` without populating it.
2. `deviceNotifier` instance might be replaced in some cases, causing `onAdded` subscribers to lose any future updates.

To fix (1), this commit added the `isPopulated` field in `deviceNotifier` as a more accurate flag to determine if we need to populate it.

To fix (2), this commit made `deviceNotifier` a final member in `PolingDeviceDiscovery`.