fix: avoid NRE in ContextMenuExtension

[gadfly3173]

Sometimes the context menu may be set to null before opening. For example, the Avalonia repository has many branches. If you right-click multiple times before the context menu of the branch is opened, NRE will appear.

[love-linger]

In fact, ContextMenuExtension.OpenContextMenu is being called in UI thread, see:

and the code of ContextMenu.Open is

        private void Open(Control control, Control placementTarget, PlacementMode placement)
        {
            if (IsOpen)
            {
                return;
            }

            if (_popup == null)
            {
                _popup = new Popup
                {
                    IsLightDismissEnabled = true,
                    OverlayDismissEventPassThrough = true,
                };

                _popup.Opened += PopupOpened;
                _popup.Closed += PopupClosed;
                _popup.Closing += PopupClosing;
                _popup.KeyUp += PopupKeyUp;
            }

            _popup.SetPopupParent(control);

            _popup.Placement = placement;

            //Position of the line below is really important. 
            //All styles are being applied only when control has logical parent.
            //Line below will add ContextMenu as child to the Popup and this will trigger styles and they would be applied.
            //If you will move line below somewhere else it may cause that ContextMenu will behave differently from what you are expecting.
            _popup.Child = this;
            _popup.PlacementTarget = placementTarget;
            _popup.HorizontalOffset = HorizontalOffset;
            _popup.VerticalOffset = VerticalOffset;
            _popup.PlacementAnchor = PlacementAnchor;
            _popup.PlacementConstraintAdjustment = PlacementConstraintAdjustment;
            _popup.PlacementGravity = PlacementGravity;
            _popup.PlacementRect = PlacementRect;
            _popup.CustomPopupPlacementCallback = CustomPopupPlacementCallback;
            _popup.WindowManagerAddShadowHint = WindowManagerAddShadowHint;
            IsOpen = true;
            _popup.IsOpen = true;

            RaiseEvent(new RoutedEventArgs
            {
                RoutedEvent = OpenedEvent,
                Source = this,
            });
        }

I think the problem may caused by these code of SourceGit

private static void OnContextMenuClosing(object sender, CancelEventArgs e)
{
    if (sender is ContextMenu menu && menu.PlacementTarget != null)
        menu.PlacementTarget.ContextMenu = null;
}

[love-linger]

BTW, this code is introduce by 0dea7ed with issue #140

[gadfly3173]

Maybe just with control.ContextMenu?.Open(); is fine, or someone has a better solution?

[love-linger]

Maybe just with control.ContextMenu?.Open(); is fine, or someone has a better solution?

I can not reproduce this issue, but I think you are right.