Failed on Testing Xamarin.Forms using UITest

I followed instruct in this link developer.xamarin.com/guides/cross-platform/xamarin-forms/uitest-and-test-cloud/ to write a test with Xamarin.Forms. I also fetched a sample from related link:
https://github.com/xamarin/xamarin-forms-samples/tree/master/UsingUITest.
I am using Visual Studio to set up this solution.
After installing required nuget such as NUnit Test Adapter for VS2012 and VS2013, included framework and Xamarin.UI.Test.
I built up the Android App and Run the test in the emulator, and I stuck in this step(first query in TestCase). The Test was failed due to it didn't find the query element as in attached image.

After using app.repl(), I figured out it seemed that the screen was take too long to load, so the query was failed.
I try using System.Threading.Thread.Sleep(10000) in "CrossPlatformTests.cs" file, at TestCase() method, before _app.Query(...) and it worked

So everyone, is it normal that I met this error, or this is truly an error? If this is an error, so could you please point out how to fix it?
Thanks very much.