detachInstance static method
Detaches platform from the shared instance slot if it currently
occupies it, so the next instance read builds a fresh backend instead
of handing out a disposed one. Platform implementations call this from
their dispose(); a no-op when platform is not the shared instance.
Implementation
static void detachInstance(BluetoothRfcommPlatform platform) {
if (identical(_instance, platform)) _instance = null;
}