title: before must be in the pushed stack of the current context
body: <p>I am creating an iOS app using Xamarin Form PCL.The flow of the Page as
  <code>Login page</code> -> <code>Page Main</code> -> <code>Page OrderList</code> -> <code>Page OrderItem</code> -><code>Page Signature</code> -> <code>Page Camera</code>.The Issue is  From Main Page to Camera Page I can come without any issue but when i try to return from Camera page to OrderList page i am getting an exception <code>before must be in the pushed stack of the current context</code>. I don't know is going wrong. Because the same code running perfectly in  Android but not running in iOS. </p>
  
  <pre><code> Navigation.InsertPageBefore(new OrderList(),  this);
   await Navigation.PopAsync();
  </code></pre>
  
  <p>complete Exception </p>
  
  <pre><code>  Message: before must be in the pushed stack of the current context
  InnerMessage: 
  StackTrace:   at Xamarin.Forms.Internals.NavigationProxy.OnInsertPageBefore (Xamarin.Forms.Page page, Xamarin.Forms.Page before) [0x00020] in D:\agent\_work\1\s\Xamarin.Forms.Core\NavigationProxy.cs:150 
    at Xamarin.Forms.Internals.NavigationProxy.InsertPageBefore (Xamarin.Forms.Page page, Xamarin.Forms.Page before) [0x00000] in D:\agent\_work\1\s\Xamarin.Forms.Core\NavigationProxy.cs:59 
    at AT.PictureAfterSignature+&lt;Process&gt;d__9.MoveNext () [0x000a2] in E:\Projects\xamrine\Source\ATiOS\AT\AT\PictureAfterSignature.xaml.cs:422 
  Source: Xamarin.Forms.Core
  </code></pre>
  
  <p>Hope someone know how to resolve this issue.</p>