Fix Illegal offset type error with Bootstrap Styles when media library selection is present

This patch checking the background image before loading it

Thanks, Mustafa, for reporting and patching.
Is this fixes the same issue in 🐛 InvalidArgumentException: Invalid ID provided for the "media" entity Active ?
Which patch fix do you think is better?

Feels that both should be committed

Released bootstrap_styles 1.1.5 →

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