Could be that the FontDidClose event is not properly catched by an observer?
It doesn’t appears either in the list when running the RoboFontExamples/observers/eventObserver.py

from mojo.events import addObserver, removeObserver


class myObserver(object):
    def __init__(self):
        addObserver(self, "didOpen", "fontDidOpen")
        addObserver(self, "didClose", "fontDidClose")
        addObserver(self, "resignCurrent", "fontResignCurrent")
        
    def didOpen(self, sender):
        print "font open!"
        
    def didClose(self, sender):
        print "font close!"
        
    def resignCurrent(self, sender):
        print "not current"
        
myObserver()