Serializing NavigationPage

I get the exception: Could not load type Xamarin.Forms.Platform._NavigationPageRenderer, Xamarin.Forms.Platform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null while decoding custom attribute, when i try to serialize a Xamarin.Forms.NavigationPage-Object

at (wrapper managed-to-native) System.MonoCustomAttrs:GetCustomAttributesInternal (System.Reflection.ICustomAttributeProvider,System.Type,bool)
  at System.MonoCustomAttrs.GetCustomAttributesBase (ICustomAttributeProvider obj, System.Type attributeType, Boolean inheritedOnly) [0x00019] in /Users/builder/data/lanes/1977/2c66d2fe/source/mono/mcs/class/corlib/System/MonoCustomAttrs.cs:103 
  at System.MonoCustomAttrs.GetCustomAttributes (ICustomAttributeProvider obj, Boolean inherit) [0x00017] in /Users/builder/data/lanes/1977/2c66d2fe/source/mono/mcs/class/corlib/System/MonoCustomAttrs.cs:284 
  at System.MonoType.GetCustomAttributes (Boolean inherit) [0x00000] in /Users/builder/data/lanes/1977/2c66d2fe/source/mono/mcs/class/corlib/System/MonoType.cs:609 
  at System.Xml.Serialization.XmlAttributes..ctor (ICustomAttributeProvider provider) [0x00032] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlAttributes.cs:64 
  at System.Xml.Serialization.XmlReflectionImporter.GetTypeNamespace (System.Xml.Serialization.TypeData typeData, System.Xml.Serialization.XmlRootAttribute root, System.String defaultNamespace) [0x00032] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlReflectionImporter.cs:454 
  at System.Xml.Serialization.XmlReflectionImporter.ImportClassMapping (System.Xml.Serialization.TypeData typeData, System.Xml.Serialization.XmlRootAttribute root, System.String defaultNamespace, Boolean isBaseType) [0x00020] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlReflectionImporter.cs:334 
  at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping (System.Xml.Serialization.TypeData typeData, System.Xml.Serialization.XmlRootAttribute root, System.String defaultNamespace) [0x00074] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlReflectionImporter.cs:215 
  at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping (System.Type type, System.Xml.Serialization.XmlRootAttribute root, System.String defaultNamespace) [0x00047] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlReflectionImporter.cs:190 
  at System.Xml.Serialization.XmlSerializer..ctor (System.Type type, System.Xml.Serialization.XmlAttributeOverrides overrides, System.Type[] extraTypes, System.Xml.Serialization.XmlRootAttribute root, System.String defaultNamespace) [0x0004d] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlSerializer.cs:227 
  at System.Xml.Serialization.XmlSerializer..ctor (System.Type type) [0x00000] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/System.XML/System.Xml.Serialization/XmlSerializer.cs:175 
  at Project.ContentPageExtension.ToXML (Xamarin.Forms.NavigationPage MyObject) [0x0000b] in /***/***/WrapperClasses.cs:22 
  at Project.App+<HandleClick>c__async0.MoveNext () [0x00f3d] in /***/***/App.cs:2389 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000b] in /Users/builder/data/lanes/1977/2c66d2fe/source/mono/mcs/class/corlib/System.Runtime.ExceptionServices/ExceptionDispatchInfo.cs:61 
  at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>m__0 (System.Object state) [0x00000] in /Users/builder/data/lanes/1977/2c66d2fe/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1006 
  at UIKit.UIKitSynchronizationContext+<Post>c__AnonStorey0.<>m__0 () [0x00000] in /Users/builder/data/lanes/1977/2c66d2fe/source/maccore/src/UIKit/UIKitSynchronizationContext.cs:24 
  at Foundation.NSAsyncActionDispatcher.Apply () [0x00000] in /Users/builder/data/lanes/1977/2c66d2fe/source/maccore/src/Foundation/NSAction.cs:164 
  at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00005] in /Users/builder/data/lanes/1977/2c66d2fe/source/maccore/src/UIKit/UIApplication.cs:63 
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0001c] in /Users/builder/data/lanes/1977/2c66d2fe/source/maccore/src/UIKit/UIApplication.cs:46 
  at Project.iOS.Application.Main (System.String[] args) [0x00008] in /***/***.iOS/Main.cs:17

right after calling

XmlSerializer xsSubmit = new XmlSerializer(typeof(Xamarin.Forms.NavigationPage));