Describe the bug

I triggered a NullReferenceException in HandleBorderThicknessDuringMaximize when I directly close the MainWindow in the Window_Loaded method.

Steps to reproduce

• Create a .net 6 wpf project
• add Mahapps Metro (currently 3.0.0-alpha0457)
• Change the Mainwindow to a MetroWindow
• attach a method to the Window Loaded event and just close it directly with:
  this.Close();

examle code

https://github.com/cocoon/ControlzExCrash

Expected behavior

Processing should skip the null object

Actual behavior

in HandleBorderThicknessDuringMaximize:
System.NullReferenceException: 'Object reference not set to an instance of an object.'

AssociatedObject = null

Environment

ControlzEx version: v6.0.0.283
Windows build number: Win11 22H2 [Version 10.0.22621.1992]
Visual Studio: 2022 17.6.0
Target Framework: .Net 6.0