osx: fix crash when modal dialog is opened over fullscreen parent.
osx: fix crash when modal dialog is opened over fullscreen parent.

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