Should use array for class attribute, not string

@thomas.frobieter: We must have been blind .....

Please review.

I'm not 100% sure about #wrapper_attributes and it's not well documented, but it looks like the 'class' property there is internally handled like a regular class attribute, which means: as array.

Before we merge and release this fix, could you perhaps test this somewhere manually and check if the classes are still applied correctly?
Otherwise, we may run into hard to find issues here, if they're gone...

Sadly I still get a "Error: [] operator not supported for strings in gin_lb_preprocess_form_element() (line 26 of /var/www/html/web/modules/contrib/gin_lb/gin_lb.module)"

when trying to edit a card "section"/layout in LB.

Reviewed with @Anybody

Automatically closed - issue fixed for 2 weeks with no activity.