applets/devicenotifier: fix crash when parent device is not of StorageDrive type
Reason for the change
If a storage volume's parent is not a storage device, the parent will not be added to m_parentDevices. When the storage volume is unmounted, we also need to check if the parent exists in m_parentDevices before using it.
Test plan
Screenshots or screen recordings
Bugs fixed
SENTRY: PLASMA-WORKSPACE-146Y FIXED-IN: 6.2.1