CDiaz-MS

Steps to reproduce: 1. Import the latest MRTK Foundation package, 2.2 2. Observe the error @andreiborodin ``` NullReferenceException: Object reference not set to an instance of an object UnityEngine.Experimental.UIElements.StyleSheets.StyleSelectorHelper.FastLookup (System.Collections.Generic.IDictionary`2[TKey,TValue]...