NullReferenceException OnClosed in LayoutAnchorableFloatingWindowControl

[Ben]

The following exception occured on a customer machine:

LayoutAnchorableFloatingWindowControl OnClosed: First Chance Exception: System.NullReferenceException:
Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.

bei AvalonDock.Controls.LayoutAnchorableFloatingWindowControl.OnClosed(EventArgs e)
bei System.Windows.Window.WmDestroy()

I don't have enough information which variable could be null perhaps _model or Manager?
I can create a pull request with nullchecks or null-conditional member access but I don't like these in my own code if I'm sure that these can never be null.
So I though I ask first which variable could be null and which style you do prefer (null Check or null-conditional member access)

[Ben]

I created a pull request for this issue