Hi Petar,

I was finally able to spend some time on creating a repro project. I reduced our application to only include parts of the navigation code and the AutoCompleteBox in a simple view. To show the difference between the two Ui for WPF versions, I attached two solutions which are identical except for the referenced library version. I hope this is of some help to identifiy the issue.

Best regards
Simon Müller

Hello Petar,

thank you for your quick response! I was not yet able to test the workaround you mentioned but will do so and report back after. However, I wanted to get back to you on the regression topic as quick as possible:

Is there any chance during your upgrade you have switched from XAML to no-XAML binaries

We are actually using the NoXaml binaries already. I can't think of any place where we would be using the Xaml ones. So I don't think any change like this could have happened.

or you have added an additional nesting in XAML, or additional Styles that apply to docking elements

This is also not the case. In fact, the View hasn't gotten any changes since October of last year.

Fortunately, I did the upgrade in a separate git commit so I was able to compare both versions. And indeed, when comparing the revision before the upgrade to 2022.1.117 with the revision after the upgrade the crash is only happening with the new version. I attached the .patch for the upgrade commit so you can also check the changes that happened.

What made me think of a regression initially is the bugfix "Loading databound AutoCompleteBox in TabControl in no-xaml binaries scenario with FluentTheme throws NullReferenceException" from the release log (which I believe is actually the fix for another problem I reported here). The scenario is pretty much the same as in this case (except that the RadTabControl is now indirectly added to the tree through the RadPaneGroup). So maybe that bug fix for the loading introduced a change that now causes this crash when unloading the control?

Regards
Simon Müller

Hello Petar,

thank you for your answer! I will try to answer your questions one by one.

> I've managed to reproduce a NUllReferenceException but with different stack trace.  [...] however, I don't believe it is related to your exception having look at your stack trace.

Indeed I think that the issue you found is not related to the issue we are encountering. We even have AutomationManager.AutomationMode = AutomationMode.Disabled; set in our App's constructor since the automation peers gave us issues in the past.

> can you send us your XAML so that we can get a picture of how is your UI configured, how are the controls nested into one another and how are they loaded - via Items properties or via MVVM with ContentTemplate /Selector properties.

Our application is quite large so I can't send you the full xaml code unfortunately. However, I will try to give you an overview of our navigation system and the important parts of the xaml code. I will also attach a small project where I tried to reproduce the issue (unfortunately I wasn't able to reproduce though...)

Regarding MVVM: Yes, we are using MVVM in most places. The only exception is our navigation code which uses Code-Behind of the MainWindow to add/remove/hide/activate RadPanes to/from our main RadDocking container (or rather the RadSplitContainer within it...).

<Grid Grid.Row="1">
<!-- Main Content -->

    <telerik:RadDocking
        x:Name="RadDocking"
        HasDocumentHost="False"
        RetainPaneSizeMode="DockingAndFloating"
        IsRestricted="True"
        Close="RadDocking_Close"
        BorderBrush="Transparent">
        <telerik:RadSplitContainer x:Name="MainContentContainer" />
    </telerik:RadDocking>
</Grid>

• All our modules are ViewModels that are set as the content of a RadPane and displayed via DataTemplates that directly map to UserControls. So in order to display "MainView" we would create a RadPane and set its Content and DataContext to an instance of MainViewModel. This implicitly loads MainView in the RadPane through a DataTemplate.
  
• One of the failing AutoCompleteBoxes is located inside the UserControl with some Grids and a ScrollViewer around the whole view. But not really a lot more (at least going up the tree).
   It is defined as follows:
  
  <telerik:RadAutoCompleteBox x:Name="CustomerNumberAutocompleteBox"
                              Grid.Row="1"
                              Grid.Column="1"
                              Margin="2,2,10,2"
                              VerticalAlignment="Center"
                              AutoCompleteMode="SuggestAppend"
                              DisplayMemberPath="CustomerNameAndNumber"
                              DropDownWidth="{Binding ElementName=CustomerNumberAutocompleteBox, Path=ActualWidth}"
                              IsEnabled="{Binding CustomerNameEnabled}"
                              ItemsSource="{Binding ViewCustomerSearchBusinessObjects, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                              SearchText="{Binding ViewCustomerNumberSearchText, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                              SelectedItem="{Binding ViewSelectedCustomerBusinessObject, ValidatesOnDataErrors=True, NotifyOnValidationError=True, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                              SelectionMode="Single"
                              TextSearchMode="Contains"
                              TextSearchPath="CustomerNumber"
                              WatermarkContent="{StaticResource Loc.CustomerNumber}">
      <i:Interaction.Behaviors>
          <behaviors:BubbleScrollBehavior /> <!-- Fix for scrolling being blocked if the mouse is over the control -->
      </i:Interaction.Behaviors>
  </telerik:RadAutoCompleteBox>

> are you able to isolate/comment the navigation view and still reproduce the exception  ?

Yes, I just commented the RadNavigationView from the XAML and the exception was still happening as before.

Additional info

• I just noticed that the issue seems to only happen for the "top" level of our module: By using the buttons in the bottom left in the initial gifs I sent you, the user is able to add positions Which are located inside a simple ItemsControl. Those positions also contain RadAutoCompleteBoxes. However, those don't seem to have the problem. See the gif I attached to this post.
  
  
• The issue also does not happen if the AutoCompleteBox's SelectionMode is set to "Multiple".
  
  
• I also did some further debugging and tried to get as much information out of the stack trace as possible. Unfortunately, all the dependencies are optimized so there was not much to get there:
  • line 7: The name of the changed property is "Content"
  • line 11 - 14: all the style stuff involves a ContentPresenter
  • line 51: The FrameWorkElement is the RadAutoCompleteBox. At this point its DataContext is null, ItemsSource is {object[0]}, SearchText is "" and SelectedItem is null
  • line 56: The BindingExpression has the Path "ViewSelectedCustomerBusinessObject" (so its for the SelectedItem)
  • line 59: The property that's invalidated seems to be "DataContext" of the RadAutoCompleteBox (makes sense since the next call goes to OnDataContextChanged())
    

So it looks like deactivating the RadPane starts a tree walk which sets the AutoCompleteBox's DataContext to null which in turn sets the SelectedItem to null, causing the Content of the AutoCompleteBox to be re-rendered (which must not happen since a tree walk is in progress).

Regards
Simon Müller