How to reproduce:
1. Open new project
2. Press Help > Manage license > Return License > Confirm > No

Actual result: the UI locks, pressing "x" on the license window results in all Unity closing.
Expected result: The UI doesn't lock, pressing "x" on license window should close only it.

Reproducible with: 2018.1.0b1, 2017.3.0f3, 2017.2.1p1, 2017.2.0a2.

Notes: You can press only "x", the menu of Unity and account settings.
Unity 2017.2.0a1 and previous versions don't show the confirmation window after pressing Confirm and returns the license.