To repro, add an action to e.g. a cube and set handler to "Set Property". Then just change the handler between different properties. Wherever you change to a property that is of different type than the previously shown property, you get a qml error like these this in log:

qrc:/Studio/Palettes/Action/HandlerProperty.qml:183:20: Unable to assign QVector3D to QString

qrc:/Studio/Palettes/Action/HandlerProperty.qml:129:20: Unable to assign QString to double

Probably happens because the model changes the type of the value property before the proper component is loaded to show the property value.

This doesn't actually seem to cause any problems, other than the cosmetic issue of getting qml errors in the log.