ui: fix crash if there is no previously open soundfont

src/widgets/file_chooser_button.rs

@@ -75,14 +75,10 @@ impl ObjectImpl for FileChooserButtonPriv {
             let priv_ = FileChooserButtonPriv::from_instance(b);
             // We must hold a reference to the Native dialog, or it crashes.
             priv_.dialog.replace(Some(file_chooser.clone()));
-            print_on_err!(file_chooser.set_current_folder(
-                priv_
-                    .file
-                    .borrow()
-                    .as_ref()
-                    .and_then(|f| f.parent())
-                    .as_ref()
-            ));
+            let current_folder = priv_.file.borrow().as_ref().and_then(|f| f.parent());
+            if current_folder.is_some() {
+                print_on_err!(file_chooser.set_current_folder(current_folder.as_ref()));
+            }
             file_chooser.connect_response(glib::clone!(@weak b => move |w, r| {
                 if r == gtk4::ResponseType::Accept {
                     b.set_property("file", &w.file());