Getting "Assertion failed on expression" error when unloading scene?

Here are the errors I'm getting:

 Assertion failed on expression: 'offsetIndex != -1'
 
 Assertion failed on expression: 'offsetIndex != -1'
 UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&
  

I'm using SceneManager.UnloadSceneAsync then loading a scene right after. This actually works perfectly, but it still throws these errors :( So it's very strange. Actually throws that second error 15 times.

I'm using basically the same script as the SceneLoader in this example project from Unity: https://github.com/UnityTechnologies/open-project-1

Has anybody run into this before? I have no idea what to do. Been very frustrating.