Reproduction steps:
1. Open Unity editor and create a new project
2. Drag UnityUIExtensions.unitypackage to Project window
3. In the Import Unity Package window select Import

Expected result: Package is imported
Actual result: Editor crashes

Reproducible with: 2017.2.0f3
Not reproducible with: 5.5.0f1, 5.6.0f1, 2017.1.2f1, 2017.2.0a4, 2017.3.0b11, 2018.1.0a7

Workaround:
Run Unity editor as administrator

Notes:
Reproducible both by drag and dropping and by right-clicking in Project window and selecting Import Package -> Custom Package