Unassigned reference exception

In my project I have a singleton class to control the UI in game, when i click a button the it will call a function inside this class to show/hide a panel, but whenever that function it's called I get a error message saying 'unassigned reference' to the panel i'm trying to hide/show, but the thing is everything works fine (at least in the editor). Here's my code:

 public IEnumerator PlayAnimation(GameObject obj, GameObject controlButton, float direction, float speed, float delay){
     yield return new WaitForSeconds(delay);
     if (obj.GetComponent<Animator>() != null) //null reference here
     {
         obj.GetComponent<Animator>().SetFloat("Direction", direction);
         if (direction > 0)
         {
             obj.GetComponent<Animator>().SetBool("Hide", true);
             controlButton.GetComponent<EventTrigger>().enabled = true;
         }
         else
         {
             obj.GetComponent<Animator>().SetBool("Hide", false);
             controlButton.GetComponent<EventTrigger>().enabled = false;
         }

         obj.GetComponent<Animator>().speed = speed;
     }
 }

 public void ShowPanel(GameObject panel, GameObject controlButton){
     StartCoroutine(uiControl.PlayAnimation(panel, controlButton, -1.0f, 1f, 0f));
     HidePanel(panel, controlButton, 3.0f);
 }

 public void HidePanel(GameObject panel, GameObject controlButton, float delay){
     StartCoroutine(uiControl.PlayAnimation(panel, controlButton, 1.0f, 1f, delay));
 }

The script of the button trying to call it:

  public void OnMouseDown(){ //using EventTrigger
     UIControl.uiControl.ShowPanel(UIControl.uiControl.topLeftPanel, UIControl.uiControl.topLeftButton);
 }