This shows the original issue on macOS/arm64 (if the rest of the PR is
not applied)
```
GLib.GException: Invalid byte sequence in conversion input
at GLib.Marshaller.FilenamePtrToString(IntPtr ptr) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/Marshaller.cs:line 62
at GLib.Marshaller.FilenamePtrToStringGFree(IntPtr ptr) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/Marshaller.cs:line 68
at Gtk.FileChooserDialog.get_Filename() in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/FileChooserDialog.cs:line 222
at Samples.FileChooserDialogDemo.OnPressed() in /Users/poupou/git/external/GtkSharp/Source/Samples/Sections/Dialogs/FileChooserDialogSection.cs:line 28
at Gtk.Button.Pressed_cb(IntPtr inst) in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/Button.cs:line 388
at GLib.ExceptionManager.RaiseUnhandledException(Exception e, Boolean is_terminal) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/ExceptionManager.cs:line 58
at Gtk.Button.Pressed_cb(IntPtr inst) in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/Button.cs:line 388
at Gtk.Application.Run() in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Application.cs:line 115
at Samples.Program.Main(String[] args) in /Users/poupou/git/external/GtkSharp/Source/Samples/Program.cs:line 44
```
