Prevent errors when accessing disposed services when project is closed

When a project is closed, the corresponding project services are disposed. If a background task is running at the same time, e.g. an analysis, it might try to access disposed services, leading to errors.

List of possible ways to fix the issue:

• Cancel background tasks when project gets closed. This will prevent errors and unnecessary work (More details on cancelation procedure here)
• Define disposable hierarchy so that services are disposed in the right order
• Try to reduce the number of services, by making some of them regular objects that other services can instantiate.