Crash when trying to export txt without opening project first

When clicking on the export button in "tracking" tab and exporting to a .txt file without opening a project first, petrack crashes.

Steps to reproduce:

1. start petrack (not via a .pet project)
2. navigate to tracking tab
3. click export
4. type in any filename ending in .txt (or any name that will cause a generation of the txt file automatically)
5. crash when exporting