[PackageManager] Editor crashes when canceling package installation

Steps to reproduce:
1. Create new project
2. Open package Manager window
3. Install any component
4. Cancel installation

Results: editor crashes when canceling package installation

Note: the crash is reproducible when canceling package removal

Reproduced with: 2018.2.0a4, 2018.1.0b13, 2018.1.0b11

Note:
PackageManager is introduced in 2018.1.0b5

Environment:
Mac(10.13,10.12), Windows(10, 7)