This is an area we’ve not changed for a while, so this bug must have been there a while! Thanks for catching it, I’ll work on a fix now.
In the mean time, you can fix your experiment by opening the .psyexp file in a text editor (e.g. notepad) and replacing this line:
<Param name="Experiment info" updates="None" val="{}" valType="code"/>
with this:
<Param name="Experiment info" updates="None" val="{'participant':''}" valType="code"/>