Fixed issue when clicking cancel on private key file picker

Steps to reproduce the issue:
- Open the Pack Item Overlay
- Click on "Browse..."
- Click on "Cancel"
- "undefined" is written into the private key file input field which makes it impossible to pack it successfully since it'll try to load this wrong private key file path.
Merge pull request #33 from beaufortfrancois/patch-3

Fixed issue when clicking cancel on private key file picker