I tested with xfce4-session 4.13 and virtualbox 6.0, and I can confirm this bug. The process does not end immediately though.
VBoxSVC.log:
00:00:25.454822 main VirtualBox: object deletion starts
00:00:25.803381 Watcher ERROR [COM]: aRC=E_ACCESSDENIED (0x80070005) aIID={d0a0163f-e254-4e5b-a1f2-011cf991c38d} aComponent={VirtualBoxWrap} aText={The object is not ready}, preserve=false aResultDetail=0
00:00:30.467997 main {00007f6e30016820} HostPowerServiceLinux::~HostPowerServiceLinux: RTThreadWait() for 5000 ms failed with VERR_TIMEOUT
00:00:30.468881 main VirtualBox: object deleted
I tested with xfce4-session 4.13 and virtualbox 6.0, and I can confirm this bug. The process does not end immediately though.
VBoxSVC.log: e254-4e5b- a1f2-011cf991c3 8d} aComponent= {VirtualBoxWrap } aText={The object is not ready}, preserve=false aResultDetail=0 eLinux: :~HostPowerServ iceLinux: RTThreadWait() for 5000 ms failed with VERR_TIMEOUT
00:00:25.454822 main VirtualBox: object deletion starts
00:00:25.803381 Watcher ERROR [COM]: aRC=E_ACCESSDENIED (0x80070005) aIID={d0a0163f-
00:00:30.467997 main {00007f6e30016820} HostPowerServic
00:00:30.468881 main VirtualBox: object deleted