Use event system instead of message system.

SphereCommandsForEvent.cs

using UnityEngine;
using UnityEngine.EventSystems;

public class SphereCommandsForEvent : MonoBehaviour, ISelectable {

    public void OnSelect() {
        if(!this.GetComponent<Rigidbody>()) {
            var rigidbody = this.gameObject.AddComponent<Rigidbody>();
            rigidbody.collisionDetectionMode = CollisionDetectionMode.Continuous;
        }
    }
}

// Add to
public interface ISelectable : IEventSystemHandler {
    void OnSelect();
}