TNG runs as a service on host PC.
The TNG service has to allocate or de-allocate a lot of memory (depending if it is starting or stopping).
So when you stop it, it has to write all of that data back to the hard drive first.
This might take some time which might cause this error.
You can actually click the "close" button after you have click stop
or start and the service will continue in the background.
You can also open your task manager or process explorer and check that
the memory usage is changing during the stop or start process.