Nigel_S on "[Plugin: WPAdverts - Classifieds Plugin] Woocommerce Addon Activation Error"

Hi,
I have just purchased the Woocommerce Addon but when I try to activate it, using the Licence Key provided, I get an error popup with just the single word "null" displayed and no activation.
Please advise.
Many thanks,
Nigel S.

https://wordpress.org/plugins/wpadverts/