Repro steps:

1. Open Unity Editor and Create a new project (e.x. Unity 5.6.2p3)
2. Download newest Unity version (e.x. 2017.1.0f3)
3. While Unity 5.6.2p3 is running, start the Unity 2017.1.0f3 installation
4. Install the software

Fixed in: 2018.1.0b1

Expected result: A prompt should appear warning the user that the Editor is open while updating and that this could cause unexpected behavior and crashes
Actual result: The 2017.1.0f3 Editor installs and the 5.6.2p3 Editor crashes

Reproduced with: 2017.2.0b3, 2017.1.0f3, 5.6.2p3, 5.5.4p2

Note:
Windows has a prompt requiring to close the Editor