GtkFileChooserButton creates native dialog for unsupported action
In the Flatpak'ed Rhythmbox, go to the import tab, and select the "Select folder..." menu item in the GtkFileChooserButton. GTK+ will try to create a native dialogue even though the GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER
action is unsupported for native dialogues.
In b3d02671, the check should also have been for gtk_file_chooser_get_action (priv->chooser) != GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER
before creating a native dialogue.