System.NullReferenceException using FlowListView inside a FlexLayout when navigating back to previous view

[chrisjcoan]

Exception

System.Exception: Cannot destroy App.Views.Page. ---> System.NullReferenceException: Object reference not set to an instance of an object at Xamarin.Forms.FlexLayout.OnChildPropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e) [0x00052] in D:\a\1\s\Xamarin.Forms.Core\FlexLayout.cs:347 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:150 at Xamarin.Forms.Element.OnPropertyChanged (System.String propertyName) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:370 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindableObject+BindablePropertyContext context, System.Object value, System.Boolean currentlyApplying, Xamarin.Forms.Internals.SetValueFlags attributes, System.Boolean silent) [0x00114] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:623 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.Internals.SetValueFlags attributes, Xamarin.Forms.BindableObject+SetValuePrivateFlags privateAttributes) [0x0015b] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:417 at Xamarin.Forms.BindingExpression.ApplyCore (System.Object sourceObject, Xamarin.Forms.BindableObject target, Xamarin.Forms.BindableProperty property, System.Boolean fromTarget) [0x00225] in D:\a\1\s\Xamarin.Forms.Core\BindingExpression.cs:173 at Xamarin.Forms.BindingExpression.Apply (System.Object sourceObject, Xamarin.Forms.BindableObject target, Xamarin.Forms.BindableProperty property) [0x0006b] in D:\a\1\s\Xamarin.Forms.Core\BindingExpression.cs:78 at Xamarin.Forms.Binding.Apply (System.Object context, Xamarin.Forms.BindableObject bindObj, Xamarin.Forms.BindableProperty tar getProperty, System.Boolean fromBindingContextChanged) [0x00057] in D:\a\1\s\Xamarin.Forms.Core\Binding.cs:130 at Xamarin.Forms.BindableObject.ApplyBindings (System.Boolean skipBindingContext, System.Boolean fromBindingContextChanged) [0x0003c] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:448 at Xamarin.Forms.BindableObject.SetInheritedBindingContext (Xamarin.Forms.BindableObject bindable, System.Object value) [0x0005a] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:135 at Xamarin.Forms.Element.SetChildInheritedBindingContext (Xamarin.Forms.Element child, System.Object context) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:504 at Xamarin.Forms.Element.<OnBindingContextChanged>b__78_0 (Xamarin.Forms.BindableObject child, System.Object bc) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:324 at Xamarin.Forms.BindableObjectExtensions.PropagateBindingContext[T] (Xamarin.Forms.BindableObject self, System.Collections.Generic.IList1[T] children, System.Action2[T1,T2] setChildBindingCo ntext) [0x0002c] in <81ae0eb290e24f558e192928e3e37a57>:0 at Xamarin.Forms.Element.OnBindingContextChanged () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:322 at Xamarin.Forms.View.OnBindingContextChanged () [0x0000c] in D:\a\1\s\Xamarin.Forms.Core\View.cs:158 at Xamarin.Forms.BindableObject.SetInheritedBindingContext (Xamarin.Forms.BindableObject bindable, System.Object value) [0x00062] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:136 at Xamarin.Forms.Element.SetChildInheritedBindingContext (Xamarin.Forms.Element child, System.Object context) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:504 at Xamarin.Forms.Element.<OnBindingContextChanged>b__78_0 (Xamarin.Forms.BindableObject child, System.Object bc) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:324 at Xamarin.Forms.BindableObjectExtensions.PropagateBindingContext[T] (Xamarin.Forms.BindableObject self, System.Collections.Generic.IList1[T] children, System.Action2[T1,T2] setChildBindingContext) [0x0002c] in <81ae0eb 290e24f558e192928e3e37a57>:0 at Xamarin.Forms.Element.OnBindingContextChanged () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:322 at Xamarin.Forms.View.OnBindingContextChanged () [0x0000c] in D:\a\1\s\Xamarin.Forms.Core\View.cs:158 at Xamarin.Forms.BindableObject.SetInheritedBindingContext (Xamarin.Forms.BindableObject bindable, System.Object value) [0x00062] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:136 at Xamarin.Forms.Element.SetChildInheritedBindingContext (Xamarin.Forms.Element child, System.Object context) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:504 at Xamarin.Forms.TemplatedPage.SetChildInheritedBindingContext (Xamarin.Forms.Element child, System.Object context) [0x00008] in D:\a\1\s\Xamarin.Forms.Core\TemplatedPage.cs:35 at Xamarin.Forms.Element.<OnBindingContextChanged>b__78_0 (Xamarin.Forms.BindableObject child, System.Object bc) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:324 at Xamarin.Forms.BindableObjectExtensions.PropagateBindingContext[T] (Xamarin.Forms.BindableObject self, System.Collections.Generic.IList1[T] children, System.Action2[T1,T2] setChildBindingContext) [0x0002c] in <81ae0eb290e24f558e192928e3e37a57>:0 at Xamarin.Forms.Element.OnBindingContextChanged () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Element.cs:322 at Xamarin.Forms.Page.OnBindingContextChanged () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Page.cs:214 at Xamarin.Forms.ContentPage.OnBindingContextChanged () [0x00000] in D:\a\1\s\Xamarin.Forms.Core\ContentPage.cs:16 at Xamarin.Forms.BindableObject.BindingContextPropertyChanged (Xamarin.Forms.BindableObject bindable, System.Object oldvalue, System.Object newvalue) [0x0000f] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:468 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindableObject+BindablePropertyContext context, System.Object value, System.Boolean currentlyApplying, Xamarin.Forms.Internals.SetValueFlags attributes, System.Boolean silent) [0x00120] i n D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:625 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.Internals.SetValueFlags attributes, Xamarin.Forms.BindableObject+SetValuePrivateFlags privateAttributes) [0x0015b] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:417 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, System.Boolean fromStyle, System.Boolean checkAccess) [0x0003d] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:573 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:99 at Xamarin.Forms.BindableObject.set_BindingContext (System.Object value) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:25 at Prism.Common.PageUtilities.DestroyPage (Xamarin.Forms.Page page) [0x0003c] in D:\a\1\s\Source\Xamarin\Prism.Forms\Common\PageUtilities.cs:46 --- End of inner exception stack trace --- at Prism.Common.PageUtilities.DestroyPage (Xamarin.Forms.Page page) [0x00046] in D:\a\1\s\Source\Xamarin\Prism.Forms\Common\PageUtilities.cs:50 at Prism.Common.PageUtilities.HandleSystemGoBack (Xamarin.Forms.Page previousPage, Xamarin.Forms.Page currentPage) [0x0002f] in D:\a\1\s\Source\Xamarin\Prism.Forms\Common\PageUtilities.cs:225 at Prism.Behaviors.NavigationPageSystemGoBackBehavior.NavigationPage_Popped (System.Object sender, Xamarin.Forms.NavigationEventArgs e) [0x00008] in D:\a\1\s\Source\Xamarin\Prism.Forms\Behaviors\NavigationPageSystemGoBackBehavior.cs:25 at Xamarin.Forms.NavigationPage+<Xamarin-Forms-INavigationPageController-RemoveAsyncInner>d__69.MoveNext () [0x00118] in D:\a\1\s\Xamarin.Forms.Core\NavigationPage.cs:309 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.iO The app has been terminated.

XAML

<FlexLayout Direction="column">
    <flv:FlowListView
                    Margin="10"
                    SeparatorVisibility="None"
                    HasUnevenRows="False"
                    HeightRequest="{Binding JobPhotos.FlowListHeight}"
                    RowHeight="{Binding JobPhotos.PhotosHeight}"
                    FlowItemsSource="{Binding JobPhotos.Photos}"
                    FlowColumnCount="{Binding JobPhotos.FlowListColumns}">
                    <flv:FlowListView.FlowColumnTemplate>
                        <DataTemplate>
                            <Image
                                Margin="10"
                                Source="{Binding ImageUrl}"
                                Aspect="AspectFill" />
                        </DataTemplate>
                    </flv:FlowListView.FlowColumnTemplate>
                </flv:FlowListView>
</FlexLayout>

[vlkam]

There is Prism also Try to remove bindings one by one to find out which binding the cause of error and then check this binding

[chrisjcoan]

I've tried removing the bindings and the app still crashes.

If I change the FlexLayout to a StackLayout, everything works fine. It's something to do with trying to set the WidthRequest OnChildPropertyChanged in the FlexLayout.cs when the view is being destroyed.

[chrisjcoan]

OK...it seems to be when the FlowListView is a child of a FlexLayout that the issue occurs. The issue is iOS specific and I can replicate it for both release and debug builds.

My current workaround is to wrap my FlowListView with a StackLayout so that the StackLayout is the child of the FlexLayout. This now works and the app no longer crashes.