DockingManager: ModelChange event happens before new LayoutDocumentItem is added

[Dirkster99]

Originally reported by https://github.com/striveprover

xceedsoftware/wpftoolkit#1430

When switched our project from .Net framework 4.6.1 to 4.7.2, the ApplyStyleToLayoutItem routine in Xceed.Wpf.AvalonDock.DockingManager causes a ModelChange event. Handling the event requires that the new LayoutDocumentItem is in the DockingManager's _layoutItems list. Unfortunately, the new LayoutDocumentItem is only added after ApplyStyleToLayoutItem completes.

The handler for ModelChange event in LayoutDocumentTabItem fails to set LayoutItem property and its CloseCommand no longer executes. Also, the item appears blank. Deselecting and reselecting allows its content to show, but does not fix the CloseCommand.

To be clear, we switched the .Net framework only in our projects and not in the Avalon dock project.

The fix appears to be to re-order the two lines (in two places) in Xceed.Wpf.AvalonDock.DockingManager.cs as shown below.

   _layoutItems.Add( layoutItem );
  ApplyStyleToLayoutItem( layoutItem );

[Dirkster99]

Issue fixed in version 3.4.11 of AvalonDock