Cant give player a reward, because function OnUnityAdsDidFinish crashes outside of editor

So I am making a simple mobile game where you have to pass a level.
If you run out of lives I want to give a player a second chance by letting them watch a rewarded video.

Problem is that when the before mentioned method is called everything works fine in editor, but on phone I get this error:

UnityException: GetComponentFastPath can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.​

I set up the UnityAdsListener from this guide

Edit:
Resolved problem by using booleans to "call" methods instead of caling a C# method directly