[MISC] Autocubemap crashes when not in a level

[Blixibon]

Describe the bug

The autocubemap utility crashes the game when a map is not loaded.

Steps to reproduce

Steps to reproduce the behavior:

1. Open the console with no map loaded.
2. Type autocubemap_start into the console.
3. See error

Expected behavior

If there is a list of maps currently loaded, the game should load the first map. If not, a warning message should appear that there is no map to build cubemaps for.