2.0: Style creation can become stuck if theme only has `references` and no active token sets

[Jan Six]

When creating styles we end up in a endless loading state when sets used in the themes are not active but disabled or reference only.

To reproduce:

• Create new file
• Create a token
• Create a theme group called Theme / Default or anything
• Set the token set to disabled or reference only
• Proceed to create styles (it's enough to leave one of the styles checkboxes active)

Expected: Do not get stuck on style creation
Actual: Style creation spinner goes on forever

Bonus: Can we adjust the error message if the reason for it was not that no themes were active but that inside there weren't any sets active? No styles created. Make sure some sets are active