NullReferenceException RenderLoop.TimerTick hiding a ContextMenu

I want to hide a context menu completely based on a property from my data source. The context menu works with the XAML below as expected but there are a lot of exceptions fired in the background by TimerTick if i do a right click on a tree view item where IsRootNode is false.

<ContextMenu IsVisible="{Binding IsRootNode}">
	<MenuItem Header="Remove"></MenuItem>
</ContextMenu

The exception shown in debugging output:

[Visual] Exception in render loop: "System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.Skia.FramebufferRenderTarget.CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
   at Avalonia.Rendering.DeferredRenderer.EnsureDrawingContext(IDrawingContextImpl& context)
   at Avalonia.Rendering.DeferredRenderer.UpdateRenderLayersAndConsumeSceneIfNeeded(IDrawingContextImpl& context, Boolean recursiveCall)
   at Avalonia.Rendering.DeferredRenderer.Render(Boolean forceComposite)
   at Avalonia.Rendering.RenderLoop.TimerTick(TimeSpan time)" (Avalonia.Rendering.RenderLoop #25871395)

The complete XAML for the TreeView containing the context menu:

<TreeView x:Name="NavigationTreeView" Grid.Row="0" Grid.Column="0" Items="{Binding TreeNodes}">
	<TreeView.DataTemplates>
		<TreeDataTemplate DataType="models:TreeNode" ItemsSource="{Binding Nodes}">
			<StackPanel Orientation="Horizontal">
				<StackPanel.ContextMenu>
					<ContextMenu IsVisible="{Binding IsRootNode}">
						<MenuItem Header="Remove"></MenuItem>
					</ContextMenu>
				</StackPanel.ContextMenu>
				<Image Source="{Binding Image}"/>
				<TextBlock Margin="5,0,0,0" Foreground="Black" Text="{Binding DisplayName}"/>
				<TextBlock Margin="10,0,0,0" Foreground="Gray" Text="{Binding SizeDisplayValue, StringFormat='({0})'}" HorizontalAlignment="Right"/>
			</StackPanel>
		</TreeDataTemplate>
	</TreeView.DataTemplates>
</TreeView>

Expected behavior No exceptions to occur.

Desktop:

• OS: Windows 10
• Version: 0.9.12
• .NET Core 3.1

[maxkatz6]

@lordzomat In your ProgressWindow you have following code:

private void Execute()
{
    worker.RunWorkerAsync();
    ShowDialog(this);
}

Where you tries to open Dialog window with itself as a owner. Window can’t own itself, you need to pass another windows as a first parameter in ShowDialog method. In your case it probably should be MainWindow. Or you can use “Show” method without “Dialog” part that doesn’t requires owner to be presented.

If it worked before, it must have been undefined behavior that is also a bug, in my opinion. On other hand, ShowDialog should throw a clear exception, if owner parameter is invalid.

[lordzomat]

@maxkatz6 thanks for the hint, that did the trick. I did a few tests and i cannot reproduce the issue with 0.10.0-rc1 so i think the issue can be closed.