@aliakbar, this is a bug. And it is not specific to 21.04. Another user had complained similarly but in his case he was trying to set Thunar as his default file manager. If you would like to file a bug please do so otherwise I'll file it in a day.
EDIT: I found a topic where @Wimpy our project leader, replied (cryptic to me). Probably that means to say it cannot be changed (?!). Someone has to confirm.