UI crashes

I am getting this error dont know why. My UI is not visible in device while it is visible on unity3D play mode
The error is
Unity.app/Contents/UnityExtensions/Unity/GUISystem/UnityEngine.UI.dll’ is in timestamps but is not known in guidmapper…!

Hi,

This is a known issue: Unity Issue Tracker - UnityEngine.UI.dll is in timestamps but is not known in guidmapper...

http://forum.unity3d.com/threads/unityengine-ui-dll-is-in-timestamps-but-is-not-known-in-assetdatabase.274492/page-3

Ok you pointed me to right direction thanks