TextBox in AdornerLayer causes collection modified exception

[BAndysc]

Describe the bug

When a TextBox with non empty Text is added to an AdornerLayer and no text box has ever been focused yet, the next Measure pass will run ApplyTemplate on the textbox, this will access the VisualLayerManager.TextSelectorLayer property, the getter will create the layer and Collection Was Modified while Enumeration exception will be thrown, because all those methods will be run from within VisualLayerManager's MeasureOverride, which iterates through its layers.

To Reproduce

Steps to reproduce the behavior:

        [Fact]
        public void TextBox_In_AdornerLayer_Will_Not_Cause_Collection_Modified_In_VisualLayerManager()
        {
            using (UnitTestApplication.Start(Services))
            {
                var button = new Button();
                var root = new TestRoot()
                {
                    Child = new VisualLayerManager()
                    {
                        Child = button
                    }
                };
                var adorner = new TextBox { Template = CreateTemplate(), Text = "a" };

                var adornerLayer = AdornerLayer.GetAdornerLayer(button);
                adornerLayer.Children.Add(adorner);
                AdornerLayer.SetAdornedElement(adorner, button);

                root.Measure(Size.Infinity);
            }
        }

Expected behavior

No crash

Environment

• Avalonia-Version: 11.1.999-cibuild0044396-beta (latest master as of today)