After last plugin update, when editing a post grid, post category list, as well as post_tag list and post_format list, are not not showed. Inside Post Taxonomies & terms area, when clicking on a small frame just on the right of the check, nothing happen (WordPress 5.1.1).
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after plugin update, you should see the notice at the top
Data update required for Post Grid » Update
Please see the screenshot
you should click on update link and the process will automatically be done.
Already created grid you will see the option “Post Taxonomies & terms” please click on each expand icon and will see the options to select categories.