Add untracked changes not working #1040

Describe the bug
After setting up git storage 'Add Untracked Changes' button returns an error

To Reproduce
Steps to reproduce the behavior:

1. Set up git storage
2. Click on 'Add Untracked Changes'
3. See error: Network Error: JSON.parse: unexpected character at line 1 column 1 of the JSON data
4. Docker logs show:

wiki    | 2019-09-17T20:29:28.709Z [MASTER] info: (STORAGE/GIT) Adding all untracked content...  
wiki    | 2019-09-17T20:29:28.711Z [MASTER] warn: page.getFileExtension is not a function

Expected behavior
All pages are added to git and synced with remote

Host Info

• OS: Docker
• Wiki.js version: 2.0.0-beta.303
• Database engine: postgres 9.6.15

Additional context
Git sync works just fine. I'm able to push and pull from command line however pages that I create in wiki.js are not added to the repo