How to print debugging messages?

See `print` call at the beginning of `actionPerformed`:

class HelloAction : AnAction("Hello") {
    override fun actionPerformed(event: AnActionEvent) {
        print("Event: $event")

        val project = event.project
        Messages.showMessageDialog(project, "Hello world!", "Greeting", Messages.getInformationIcon())
    }
}

This is not printed when the action is performed, but after I stop the plugin! What do I have to do to see the output immediately?