When the D-Bus daemon terminates, the local D-Bus library sends a Disconnected signal to the application. The application can listen for this signal like any other D-Bus signal and appropriate actions can be taken.
No filter is necessary to receive this signal.
For example, to terminate the application with an exit code of 99 when the D-Bus disappears:
dbus listen [dbus info local] Disconnected {apply {args {exit 99}}}