@Dudiing@Poka Thank you for your report! We would like to look into this further, would you be able to submit a bug report using the form available here? Fortnite Creative
Thank you soooo much for this workaround using “Add Selected Asset from Content Browser”! It appears that this problem hasn’t been fixed yet but I’m still able to add custom widgets now, using your method. Multiple people are reporting this so hopefully it gets fixed soon. I’m upvoting all of them.