To reproduce:

1. Open Unity and create the new project
2. When project loads, open Asset Store
3. Find "Unity Ads", download it and Import it
4. Observe that it throws "Opening file failed" window
5. Press "Cancel: button

Expected Result: Unity Editor successfully cancel the process
Actual Result: Unity Editor crashes

Notes:
- This issue appears only on Windows
- The stack trace is shared in the comments section
- Workaround: If later the project crashes on opening or you cannot import "Unity Ads", just download Unity 2017.4 or open in the older version and everything will succeed

Reproduced on Unity 2018.1.0b5, 2018.1.0b13 and 2018.2.0a8
Not reproduced on Unity 2017.3.2f1, 2017.4.1f1 and 2018.1.0b4
Regression on Unity 2018.1.0b5