GtkFlowBoxChild is missing from widget list
GtkFlowBox can be added to the UI from the widget list, but trying to add widgets to it results in an error popup with the text 'Only objects of type Flow Box Child can be added to objects of type Flow Box'.
GtkFlowBoxChild is not in the widget list, so nothing can be added to the FlowBox in Glade.
When the FlowBoxChild and a placeholder under it are added by manually editing the XML file, GtkFlowBox works fine in Glade.
I'm running Glade 3.38.2 from Debian bullseye.