Xplayer: 'number of screenshots' field wont accept input

[Andy]

Linux Mint 18.2 Sonya - Cinnamon 64 bit

If you load a movie, then select 'Edit', then 'Create Screenshot Gallery' you reach a page which allows gallery details to be specified.

If you 'un-tick' the checkbox labelled 'Calculate the number of Screenshots' the field below (labelled 'Number of Screenshots') remains greyed out and won't accept input. Logically if the software isn't being asked to decide the number of screenshots then the user must specify it - this is how it used to work in Totem and presumably should work in Xplayer. This is entirely reproducible, happens every time and has been the same for at least a year.

Thanks