EditorApplication.OpenScene causing NullReferenceException

I'm creating an editor window that includes functions for opening scenes. It's working and the scenes open, but I'm getting null reference errors that I can't get rid of:

 NullReferenceException: Object reference not set to an instance of an object
 UnityEditor.DockArea.EndOffsetArea ()
 UnityEditor.DockArea.OnGUI ()

Anyone know how to fix this?

Below is a script I've tested in a new project. The test scenes have nothing in them except a cube and a sphere.

 class TestWindow extends EditorWindow {
     @MenuItem("Window/TestWindow")
     static function Init() {
         var window : TestWindow = EditorWindow.GetWindow(TestWindow);
     }
 
     function OnGUI() {
         var openTest1 : boolean = false;
         EditorGUILayout.BeginHorizontal();
         if(GUILayout.Button("Test1", EditorStyles.toolbarButton, GUILayout.Width(100))) {
             openTest1 = true;
             //EditorApplication.SaveCurrentSceneIfUserWantsTo();
             //EditorApplication.OpenScene("Assets/Test1.unity");
             //return;
         }
         if(GUILayout.Button("Test2", EditorStyles.toolbarButton, GUILayout.Width(100))) {
             EditorApplication.SaveCurrentSceneIfUserWantsTo();
             EditorApplication.OpenScene("Assets/Test2.unity");
             return;
         }
         EditorGUILayout.EndHorizontal();
 
         if(openTest1) {
             EditorApplication.SaveCurrentSceneIfUserWantsTo();
             EditorApplication.OpenScene("Assets/Test1.unity");
         }
     }
 }