title: Suspending Windows Store App: Error when calling MvxSuspensionManager.SaveAsync()
body: <p>I have a WinRT app using MvvmCross where I want to store the current session state.</p>
  
  <p>In the OnLaunch method of tha App.xaml.cs I register the content frame of my app with the suspension manager:</p>
  
  <pre><code>...
  var setup = new Setup(_contentFrame);
  setup.Initialize();
  
  var suspensionManager = Mvx.GetSingleton&lt;IMvxSuspensionManager&gt;() as MvxSuspensionManager;
  suspensionManager.RegisterFrame(_contentFrame, "FrameNav");
  ...         
  </code></pre>
  
  <p>Whenever I suspend the app and run the code in the OnSuspending event handler </p>
  
  <pre><code>var suspensionManager = Mvx.GetSingleton&lt;IMvxSuspensionManager&gt;() as MvxSuspensionManager;
  
  await suspensionManager.SaveAsync();
  </code></pre>
  
  <p>I get the following exception.</p>
  
  <pre><code>Cirrious.MvvmCross.WindowsStore.Views.Suspension.MvxSuspensionManagerException "MvxSuspensionManager failed"
  at Cirrious.MvvmCross.WindowsStore.Views.Suspension.MvxSuspensionManager.&lt;SaveAsync&gt;d__0.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
  at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
  at ****App.xaml.cs:line 129
  System.Runtime.InteropServices.COMException "Unbekannter Fehler
  "
  at Windows.UI.Xaml.Controls.Frame.GetNavigationState()
  at Cirrious.MvvmCross.WindowsStore.Views.Suspension.MvxSuspensionManager.SaveFrameNavigationState(Frame frame)
  at Cirrious.MvvmCross.WindowsStore.Views.Suspension.MvxSuspensionManager.&lt;SaveAsync&gt;d__0.MoveNext()
  </code></pre>
  
  <p>I am pretty sure I can narrow the exception root to the WinRT API call to </p>
  
  <pre><code>Frame.GetNavigationState();
  </code></pre>
  
  <p>that is executed by the MvxSuspensionManager. When I call this function directly it will fail with the same error.</p>
  
  <p>I know the WinRT frame navigation stack can only be serialized if non complex types are used as navigation parameters. But my app crashes even if I just suspend on the first page.</p>
  
  <p>Maybe my approach of suspending with MvvmCross is wrong or I missed some steps to be setup correctly.</p>
  
  <p>Any help is highly appreciated!</p>