qt - QML drops binding after assignment `undefined` -

i have pyqt5 property proper notify-function:

class friendswidgetbackend(qobject):     messagecompanionchanged = pyqtsignal()      def __init__(self, parent=none):         qobject.__init__(self, parent)         self._current_message_companion = none      @pyqtproperty('qvariant', notify=messagecompanionchanged)     def message_companion(self):         return qvariant(self._current_message_companion)      @message_companion.setter     def message_companion(self, message_companion):         self._current_message_companion = message_companion         self.messagecompanionchanged.emit() 

everything works until i'll assign variable undefined - when refuses update variable in qml after emiting messagecompanionchanged signal. i've tried set breakpoint - updates application start until undefined assignment:

item {     id: rootitem      property qtobject friendswidgetbackend      onvisiblechanged: {         if (!visible) {             friendswidgetbackend.message_companion = undefined         }     } } 

i'd after experience of c# qml disconnects binding after such assignment. how can avoid behavior?

p.s. binding example:

column {     visible: friendswidgetbackend.message_companion ? true : false } 

p.p.s. not work without binding - tried connect messagecompanionchanged signal qml - not update value after emitting.

p.p.p.s. if change undefined empty js object {} or ({}) binding work not need.