Steps to reproduce:

1. Create a campaign
2. Have a character in the campaign with no class selected
3. Go to the encounter builder and build an encounter
4. Under player characters, select the preset for the campaign with the "level 0" character in it
5. Attempt to save the encounter

Expected behavior:

Either it lets you save, or there is a verbose error explaining what the issue is

Actual behavior:

There is a generic error advising you to refresh and try again.

Workaround:

Ensure that all of your characters have a class selected.