I had a project on Unity and I used 2020 version of Unity. Then my colleague pulled my project from the github and tried to open it with 2021 version of Unity. The project was redone for the 2021 version and everything seemed to work well. Then he tried to close Unity, as usual a quitting window came out. I said it usually doesn't take very long. But it turned out that it took a long time. He decided to leave it for the sake of experiment and so it worked for about 70 hours. What could have gone wrong? Why is Unity not closing?