The issue upon Replacing the WordPress Post Editor

When I use the functionality of https://piklist.com/learn/doc/replacing-wordpress-post-editor/ , the editor does not save the data. Along with this I am getting errors https://prntscr.com/d271rc . The same errors appear in demo data created by the plugin. I have tried to turn off all other plugins – the errors still remain. Could you please advice what can be the cause and how can I resolve this issue? Thanks