InvalidOperationException: This can only be used during play mode, please use EditorSceneManager.OpenScene() instead.

Hi everybody. I was experiencing this error on 2017 and I updated to 2018 to find it persists.

InvalidOperationException: This can only be used during play mode, please use EditorSceneManager.OpenScene() instead.

When I double click on the console log it takes me to the following line of code: Application.LoadLevel(sceneIndex); I am well aware that it is deprecated but using Scenemanager.LoadScene gives me exactly the same error.

Below the class it's getting the error.

     public class Scener : MonoBehaviour {
         public static Scener _instance;
         public static Scener Instance
         {
             get
             {
                 if (_instance == null)
                 {
                     _instance = new GameObject("scener").AddComponent<Scener>();
                 }
                 return _instance;
             }        
         }
     
         public void LoadScene(int sceneIndex)
         {
             Application.LoadLevel(sceneIndex);
         }
     }

Any help would be greatly appreciated!