Xamarin.forms: Null reference exception on some Android devices - Xamarin.Forms.Platform.Android.PageRenderer.Xamarin.Forms.Platform.Android.IOrderedTraversalController.UpdateTraversalOrder

Created on 2 Aug 2019 · 6Comments · Source: xamarin/Xamarin.Forms

Description

A null reference exception sometimes occur in Xamarin.Forms.Platform.Android.PageRenderer.Xamarin.Forms.Platform.Android.IOrderedTraversalController.UpdateTraversalOrder.

Affected platforms: Android
Affected build mode: Debug, Release
Affected devices:
- Xiaomi Redmi 6
- Xiaomi Mi 8
- Samsung S8
Unaffected devices:
- Emulators
- Xiaomi Redmi Note 7
- LG G6
- Google Nexus 5
- Sony Xperia
- Motorola G6
- Xiaomi Redmi 4X
- Nokia 8

Steps to Reproduce

using Xamarin.Forms;

namespace TestCSharpXamarinForms
{
    public partial class App : Application
    {
        private bool _flag = true;

        public App()
        {
            InitializeComponent();

            var button = new Button { Text = "Continue" };
            button.Clicked += (_, __) =>
            {
                var view = _flag ? (View)new Entry { Text = "Press 1 time to crash" } : new Label { Text = "I'm shown, bug fixed!" };
                _flag ^= true;
                ((MainPage as ContentPage).Content as StackLayout).Children[1] = view;
            };
            MainPage = new ContentPage
            {
                Content = new StackLayout
                {
                    Children =
                    {
                        button,
                        new Label { Text = "Press 2 times to crash" }
                    }
                }
            };
        }
    }
}

Expected Behavior

I'm shown, bug fixed! is shown after 2 presses on Continue.

Actual Behavior

System.NullReferenceException: Object reference not set to an instance of an object.
  at Xamarin.Forms.Platform.Android.PageRenderer.Xamarin.Forms.Platform.Android.IOrderedTraversalController.UpdateTraversalOrder () [0x000a4] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\PageRenderer.cs:177 
  at Xamarin.Forms.Platform.Android.VisualElementRenderer`1[TElement].UpdateParentPageTraversalOrder () [0x00025] in D:\a\1\s\Xamarin.Forms.Platform.Android\VisualElementRenderer.cs:404 
  at Xamarin.Forms.Platform.Android.VisualElementRenderer`1[TElement].OnElementPropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e) [0x00116] in D:\a\1\s\Xamarin.Forms.Platform.Android\VisualElementRenderer.cs:369 
  at Xamarin.Forms.Platform.Android.ViewRenderer`2[TView,TNativeView].OnElementPropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.Android\ViewRenderer.cs:188 
  at Xamarin.Forms.Platform.Android.EntryRendererBase`1[TControl].OnElementPropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e) [0x002db] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\EntryRenderer.cs:251 
  at (wrapper delegate-invoke) <Module>.invoke_void_object_PropertyChangedEventArgs(object,System.ComponentModel.PropertyChangedEventArgs)
  at Xamarin.Forms.BindableObject.OnPropertyChanged (System.String propertyName) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:211 
  at Xamarin.Forms.Element.OnPropertyChanged (System.String propertyName) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:353 
  at Xamarin.Forms.Element.set_Parent (Xamarin.Forms.Element value) [0x00094] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:210 
  at Xamarin.Forms.Element.OnChildRemoved (Xamarin.Forms.Element child) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:335 
  at Xamarin.Forms.VisualElement.OnChildRemoved (Xamarin.Forms.Element child) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\VisualElement.cs:750 
  at Xamarin.Forms.Layout`1[T].OnChildRemoved (Xamarin.Forms.Element child) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Layout.cs:41 
  at Xamarin.Forms.Layout.OnInternalRemoved (Xamarin.Forms.View view) [0x00012] in D:\a\1\s\Xamarin.Forms.Core\Layout.cs:464 
  at Xamarin.Forms.Layout.InternalChildrenOnCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x0002b] in D:\a\1\s\Xamarin.Forms.Core\Layout.cs:427 
  at (wrapper delegate-invoke) <Module>.invoke_void_object_NotifyCollectionChangedEventArgs(object,System.Collections.Specialized.NotifyCollectionChangedEventArgs)
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00018] in <ca7419b40e504a6dbe088f6fe95d09aa>:0 
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedAction action, System.Object oldItem, System.Object newItem, System.Int32 index) [0x0000b] in <ca7419b40e504a6dbe088f6fe95d09aa>:0 
  at System.Collections.ObjectModel.ObservableCollection`1[T].SetItem (System.Int32 index, T item) [0x0001c] in <ca7419b40e504a6dbe088f6fe95d09aa>:0 
  at System.Collections.ObjectModel.Collection`1[T].set_Item (System.Int32 index, T value) [0x0002b] in <ff07eae8184a40a08e79049bbcb31a0e>:0 
  at Xamarin.Forms.ObservableWrapper`2[TTrack,TRestrict].set_Item (System.Int32 index, TRestrict value) [0x00029] in D:\a\1\s\Xamarin.Forms.Core\ObservableWrapper.cs:140 
  at TestCSharpXamarinForms.App.<.ctor>b__1_0 (System.Object _, System.EventArgs __) [0x0003d] in C:\Users\Admin\source\repos\XamarinFormsNRE\TestCSharpXamarinForms\App.xaml.cs:18 
  at Xamarin.Forms.Button.Xamarin.Forms.Internals.IButtonElement.PropagateUpClicked () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Button.cs:177 
  at Xamarin.Forms.ButtonElement.ElementClicked (Xamarin.Forms.VisualElement visualElement, Xamarin.Forms.Internals.IButtonElement ButtonElementManager) [0x0001f] in D:\a\1\s\Xamarin.Forms.Core\ButtonElement.cs:61 
  at Xamarin.Forms.Button.SendClicked () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Button.cs:163 
  at Xamarin.Forms.Platform.Android.ButtonElementManager.OnClick (Xamarin.Forms.VisualElement element, Xamarin.Forms.IButtonController buttonController, Android.Views.View v) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.Android\ButtonElementManager.cs:25 
  at Xamarin.Forms.Platform.Android.FastRenderers.ButtonRenderer.Android.Views.View.IOnClickListener.OnClick (Android.Views.View v) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.Android\FastRenderers\ButtonRenderer.cs:71 
  at Android.Views.View+IOnClickListenerInvoker.n_OnClick_Landroid_view_View_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_v) [0x00011] in <a2f1be6565a84d5393d8f9efa19f479b>:0 
  at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.42(intptr,intptr,intptr)