prevent segfault when closing app with child windows open.
prevent segfault when closing app with child windows open.

SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,grokys/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,grokys/Perspex,AvaloniaUI/Avalonia