Steps to repro:

1. Open attached project and 't1' scene;
2. Select Terrain gameobject in the Hierarchy;
3. In Terrain Inspector go to 'Paint Texture' tab;
4. In the pallette select second white texture;
5. Try to paint white texture on terrain with using one of terrain brushes.

Expected result:
Texture is painted on terrain.

Actual result:
- Texture is not painted;
- Error:

Exception: Float array size wrong (layers should be 2)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:238)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:407)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:103)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Event evt, UnityEngine.Experimental.UIElements.VisualElement finalTarget) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:140)
UnityEngine.Experimental.UIElements.EventDispatcher.DispatchEvent (UnityEngine.Event e, UnityEngine.Experimental.UIElements.IVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/EventDispatcher.cs:226)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (UnityEngine.Experimental.UIElements.IVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:214)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:67)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:186)

is thrown. See attached video.

Notes:
- Reproducible in 2017.1.0b3, 2017.1.0b2;
- Reproducible only for new .NET 4.6 / not reprodicuble under .NET 3.5;
- Reproducible on Windows 10 and OSX 10.12.3;
- The bug doesn't affect painting of grass and detail meshes.