I will also go with Anna's suggestion with some small changes. Like
1. Instead of using boolean variable to see if validation is executed or not use it if field is valid or not. So its name could be IsValid, default value to true. In your action if SelectedItem is empty set it to false.
2. Instead ot wrapping your validation message in if widget simpaly set its display/ visible property to show/ hide based on the IsValid variable. No need to check SelectedItem is empty or not.
I am suggesting these changes because if includes, less assignment operation and conditional check :)