Generating game object in random position

Hi, I'm currently trying to get a gem to be picked up, dissappear, and respawn somewhere else random on the board. The game is 2D.

I'm having trouble with the game saying "UnassignedReferenceException: The variable Gem of PlayerMovement has not been assigned."

Here is my code

using UnityEngine; using System.Collections;

public class PlayerMovement : MonoBehaviour {

 public float speed;
 public GameObject Gem;
 Vector2 RandomSpawn = new Vector2 (Random.Range(-4.5F,4.5F), Random.Range(-9.5F,9.5F));
 private Rigidbody2D rb2d;

 void Start ()
 {
     rb2d = GetComponent<Rigidbody2D> ();
 }
 

 void FixedUpdate () 
 {
     float moveHorizontal = Input.GetAxis ("Horizontal"); //Gets keys that unity refers to being able to move horizontal (Set by default)
     float moveVertical = Input.GetAxis ("Vertical"); //Gets keys that unity refers to being able to move vertical (Set by default) Use forces to act with RigidBody2D
     Vector2 movement = new Vector2 (moveHorizontal, moveVertical);
     rb2d.AddForce (movement * speed);
 }
 void OnTriggerEnter2D(Collider2D other)
 {
     StartCoroutine ("RespawnGem");
 }
 IEnumerator RespawnGem()
 {
     if (gameObject.CompareTag ("pickup"))
         Destroy (Gem.gameObject);
     Instantiate (Gem, RandomSpawn, Quaternion.identity);
     yield return null;
 }

}

I've only started doing this recently, anything is appreciated.

Thanks.