Null Reference exceptions are continuously thrown when importing interactive tutorial.

Steps to repro:
1. Open Unity Hub.
2. Go to the Learn section.
3. Download any tutorial > start.
4. Resolve error in the console (MinAttribute).

Actual Result:
Unable to start the tutorial and Errors appear continuously.

Expected Result:
Tutorial starts.

Reproducible on:
2019.1.0a12, 2019.1.0a8

Working fine:
2019.1.0a7,2018.3.0f2

Environment:
Windows and Mac.

Stack Trace:
NullReferenceException: Object reference not set to an instance of an objectUnity.InteractiveTutorials.UIElementsHelper.GetVisualTree (Unity.InteractiveTutorials.GUIViewProxy guiViewProxy) (at <2aea49b6a2bc47eeaad333d0db921851>:0)Unity.InteractiveTutorials.MaskingManager.Mask (Unity.InteractiveTutorials.UnmaskedView+MaskData unmaskedViewsAndRegionsMaskData, UnityEngine.Color maskColor, Unity.InteractiveTutorials.UnmaskedView+MaskData highlightedRegionsMaskData, UnityEngine.Color highlightColor, System.Single highlightThickness) (at <2aea49b6a2bc47eeaad333d0db921851>:0)Unity.InteractiveTutorials.TutorialWindow.ApplyMaskingSettings (System.Boolean applyMask) (at Assets/Framework/Interactive Tutorials/Editor/Editor Windows/TutorialWindow.cs:683)Unity.InteractiveTutorials.TutorialWindow.ApplyQueuedMask () (at Assets/Framework/Interactive Tutorials/Editor/Editor Windows/TutorialWindow.cs:351)UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at C:/buildslave/unity/build/Editor/Mono/EditorApplication.cs:200)