[GTK3] More changes to workaround SafeHandle not being allowed to be null

src/Gtk/Avalonia.Gtk3/Interop/Utf8Buffer.cs

@@ -11,6 +11,8 @@ namespace Avalonia.Gtk3.Interop
             
         public Utf8Buffer(string s) : base(IntPtr.Zero, true)
         {
+            if (s == null)
+                return;
             _data = Encoding.UTF8.GetBytes(s);
             _gchandle = GCHandle.Alloc(_data, GCHandleType.Pinned);
             handle = _gchandle.AddrOfPinnedObject();

src/Gtk/Avalonia.Gtk3/SystemDialogs.cs

@@ -18,7 +18,8 @@ namespace Avalonia.Gtk3
             bool multiselect, string initialFileName)
         {
             GtkFileChooser dlg;
-            using (var name = title != null ? new Utf8Buffer(title) : null)
+            parent = parent ?? GtkWindow.Null;
+            using (var name = new Utf8Buffer(title))
                 dlg = Native.GtkFileChooserDialogNew(name, parent, action, IntPtr.Zero);
             if (multiselect)
                 Native.GtkFileChooserSetSelectMultiple(dlg, true);
@@ -78,14 +79,14 @@ namespace Avalonia.Gtk3
 
         public Task<string[]> ShowFileDialogAsync(FileDialog dialog, IWindowImpl parent)
         {
-            return ShowDialog(dialog.Title, ((WindowBaseImpl) parent)?.GtkWidget ?? GtkWindow.Null,
+            return ShowDialog(dialog.Title, ((WindowBaseImpl) parent)?.GtkWidget,
                 dialog is OpenFileDialog ? GtkFileChooserAction.Open : GtkFileChooserAction.Save,
                 (dialog as OpenFileDialog)?.AllowMultiple ?? false, dialog.InitialFileName);
         }
 
         public async Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
         {
-            var res = await ShowDialog(dialog.Title, ((WindowBaseImpl) parent)?.GtkWidget ?? GtkWindow.Null,
+            var res = await ShowDialog(dialog.Title, ((WindowBaseImpl) parent)?.GtkWidget,
                 GtkFileChooserAction.SelectFolder, false, dialog.InitialDirectory);
             return res?.FirstOrDefault();
         }