Thanks for the report Bart. This is now fixed in 1.0.1.b4 in the downloads area. The same problem was actually causing a few speed issues in the plugins area in WordPress too, so this is good to have fixed.
MasterPress 1.1.7 is now available. Another minor release. Fixing a compatibility issue with Gutenberg and the Visual Editor field. We’ve also added a quick note under the empty license key field in the plugin settings, to avoid people using their limited license keys on development websites.
MasterPress requires a minimum of WordPress version 4.9, MySQL 5.6, and PHP version 5.6.20.
We also recommend that PHP is configured to use a memory limit of 64MB per request (128MB may be required for sites with higher complexity).
This plug-in is not compatible with the WordPress.com hosted service.