Since you are unable to reproduce it, I have looked into it a bit, to try and reproduce it myself. I have a wordpress multisite installation, so different sites are running different themes and are using different plugins.
I could not reproduce it on one site, but am now trying another site (using the 2012 theme) and have got the error in my logs. I will try and narrow down the exact combination of theme & plugins, and post back when I have more info. (Might be tomorrow now).
Thanks for the effort @djeyewater! I will now try to reproduce it here. I hope the issue is not in your production server as you are not supposed to turn on Notices there. I will try to provide you a quick fix today and that will be later integrated into the upcoming plugin update.