Fix Error 500 when one user attempts to access another's personal, internal snippet

### What does this MR do?

This MR fixes an Error 500 that occurred if one user tried to access another's personal, internal snippet. Steps to reproduce:

### Why was this MR needed?

1. Go to `<hostname>/snippets/new`.
2. Select "Internal".
3. Create a snippet. Save the URL (e.g. `<hostname>/snippets/20`)
4. Logout and sign in as another user.
5. Go to the URL in step 3.

### What are the relevant issue numbers?

Closes #1815

See merge request !854