missing component exception even though the component is there?

You can't guarantee which component will be destroyed first, so the error probably occurs when the rigidbody is destroyed before the ConstantForce.

It's not very clean, but you could check if the ConstantForce is destroyed before you destroy the rigidbody. You would need to check every Update.

 Update() {
   if(GetComponent(typeof(ConstantForce)) == null) {
     Destroy(rigidbody);
   }
 }

Although, if suits your needs it may be simpler just to disable the ConstantForce component rather than destroy it. I think enabling/disabling is instant.

 GetComponent(typeof(ConstantForce)).enabled = false;
 Destroy(rigidbody);