Custom Domains Drop after changing plan

Hi every!

Can anyone help me with dropping domain in shinyapps.io?

I had a "professional" plan with custom domain before.
After failed billing the plan was changed to "free" and I wanted to activate "standard plan" (without custom domain).
But this action gives an error related to custom domain.

How can I drop custom domain without switching back and forth to "professional" plan and paying extra?