MissingReferenceException: The variable material of 'UIAtlas' doesn't exist anymore.
You probably need to reassign the material variable of the 'UIAtlas' script in the inspector.
UnityEngine.Material.get_mainTexture ()
UIAtlas.Upgrade () (at Assets/NGUI/Scripts/UI/UIAtlas.cs:402)
UIAtlas.GetSprite (System.String name) (at Assets/NGUI/Scripts/UI/UIAtlas.cs:217)
UIAtlasInspector.OnInspectorGUI () (at Assets/NGUI/Scripts/Editor/UIAtlasInspector.cs:95)
UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty)
UnityEditor.DockArea:OnGUI()