Regardless whether a Bluetooth device with a battery was disconnectedfrom within an operating system (e.g. by clicking Disconnect button inthe applet) or externally (such as falling asleep on its own, orflipping a physical switch OFF), whenever theSolid::DeviceNotifier::deviceRemoved signal is emitted, thecorresponding Solid::Device's backend object has already beendestroyed; so that Core::onDeviceRemoved handler attempted todisconnect from a null pointer which produces a warning.
Given that right before emitting the deviceRemoved() signal, thesetBackendObject(nullptr) is called which literally deletes thedevice's backend object, there is no need to disconnect.