title: ViewModel is null during ViewDidLoad
body: <p>I am getting started with MvvmCross in iOS.</p>
  
  <pre><code>public class MainView : MvxTabBarViewController
  {
      public override void ViewDidLoad()
      {
          base.ViewDidLoad();
  
          var vm = (MainViewModel)this.ViewModel;
          if (vm == null)
              return;
      }
  }
  </code></pre>
  
  <p>Setting a breakpoint to the line where access the <code>ViewModel</code>, shows me, that <code>ViewModel</code> is null.</p>
  
  <p>I can workaround this by calling <code>ViewDidLoad()</code> in the constructor. Then, <code>ViewModel</code>  is null during the constructor call, but valid in the default <code>ViewDidLoad</code> call. But that looks like a workaround. can anybody help?</p>