OnRebuildRequested exception

Since I upgraded some of my scenes are now showing another exception.

I have no idea what causes this. Does anyone?

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UI.Graphic.OnRebuildRequested () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Graphic.cs:399)
UnityEngine.UI.GraphicRebuildTracker.OnRebuildRequested () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/GraphicRebuildTracker.cs:33)
UnityEngine.CanvasRenderer.RequestRefresh () (at C:/buildslave/unity/build/artifacts/generated/common/modules/CanvasRendererBindings.gen.cs:244)

I switched my build settings to a different platform (from iOS to Android). This caused the assets to be reimported and that fixed it for me.