Xamarin Ui Test IApp C# "Object reference not set to an instance of an object."

I am new using Xamarin UI Test and I am trying test a app developed by other company.

In my first .cs I am using a if to identify a condition and start my tests. So condition identified my test goes to another class and... fail.

I receive a error message "Message: System.NullReferenceException : Object reference not set to an instance of an object". When I go to line/.cs I see this line: "app.Query("Welcome to").Any();".

And in the new class I have "IApp app" with a green line below with the message "(field) IApp ZZZZZZZ.app

Field 'ZZZZZZZ.app" is never assigned to, and will always have its default value null."

I don't know how to continue my tests using "IApp app" in another .cs without using AppInitializer every time.

Any tips?