What you're seeing is indeed a bug in libmessaging-menu. Gio's g_menu_get_item_attribute doesn't allow '&' in the variant format string anymore. Fix is coming.
By the way, I changed gwibber to not always show all streams in the messaging menu, but only the ones that have new messages. The "old" way of showing all streams all the time doesn't work anymore because libmessaging-menu removes sources as soon as they are activated. Thus, the activate callback calling the mmapp.remove_source in your example is redundant. Sorry that this isn't documented very well yet.
What you're seeing is indeed a bug in libmessaging-menu. Gio's g_menu_ get_item_ attribute doesn't allow '&' in the variant format string anymore. Fix is coming.
By the way, I changed gwibber to not always show all streams in the messaging menu, but only the ones that have new messages. The "old" way of showing all streams all the time doesn't work anymore because libmessaging-menu removes sources as soon as they are activated. Thus, the activate callback calling the mmapp.remove_source in your example is redundant. Sorry that this isn't documented very well yet.