A resolution, rule or formal action of any kind is invalid unless adopted in an open meeting of the public body. A resolution, rule or formal action adopted in an open meeting that results from deliberations in a meeting not open to the public is invalid unless the deliberations were for a purpose specifically authorized in Section 206.06 and conducted at an executive session held in compliance with Section 206.06. A resolution, rule or formal action adopted in an open meeting is invalid if the public body that adopted the resolution, rule, or formal action violated Section 206.05.