@Wyndham - that error is an indication that your Generate Cover json file has somehow got corrupted, or else the upgrade from an earlier version failed. Given that you are the only person reporting this in two months since I last released this plugin it is either something specific to you or you have managed to hit a corner case that over 12,000 other users have not...
It could be revealing to look at your Generate Cover.json file, but tbh I'm just not that bothered
. I will push a release shortly that will more gracefully ignore the error so you don't have to edit your file to workaround it.