NullReferenceException in UnityEditor.InspectorWindow.GetEditorsWithPreviews (UnityEditor.Editor[] editors)

I made a CustomEditor that displays a button in the inspector that allows to open another scene. If I click that button it does open the correct scene, but the console reports a NullReferenceException with the following stack trace:

NullReferenceException: Object reference not set to an instance of an object UnityEditor.InspectorWindow.GetEditorsWithPreviews (UnityEditor.Editor[] editors) UnityEditor.InspectorWindow.DrawPreviewAndLabels () UnityEditor.InspectorWindow.OnGUI () System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)

I expect this is because I am triggering the scene load from inside the OnInspectorGUI function. I tried delaying the call to the end of OnInspectorGUI, but that doesn't fix my problem. What I probably need to do is call the function outside of any gui function. Is there a way to do that with the Button still being shown in the inspector?