Unassigned reference exception after rewarded ad

Hi. I wanted to add some rewarded ads into my simple app and I have a "AD Panel" that shows when the ad is ready. But after I click on the ad to play, I get a unassigned reference exception of that "AD Panel" which I'm trying to close after rewarding. Am I missing something or do I have to somehow assign the panel again when I am rewarding the player?

EDIT: I added debug message "AD was rewarded" into OnUnityAdsDidFinish function and I noticed that the message shows twice. But I don't know where could be the second object with the script... There wasn't one before.

Main parts of the code that are relevant to this:

 public void AddLife()
     {
         if (Advertisement.IsReady("rewardedVideo"))
         {
             AdPanel.SetActive(true);
         }
         else { End_Game(); }
     }
 
 public void ShowAD()
     {
         Advertisement.Show("rewardedVideo");
     }
 
 public void OnUnityAdsDidFinish(string placementId, ShowResult showResult)
     {
         if(showResult == ShowResult.Finished)
         {
             AdPanel.SetActive(false);
         }
         else if(showResult == ShowResult.Skipped)
         {
         }
         else if(showResult == ShowResult.Failed)
         {
             Debug.Log("Ad Failed To Finish!");
         }
     }

Thanks for ideas, if you have any.