Validation failed after udpate to v13.0.1

[julianCast]

After updating my theme from 7 to 13.0.1 I'm getting this error:

Error: templates/index.json, Validation failed: Setting 'heading' is invalid. All top level nodes must be '<p>', '<ul>', '<ol>' or '<h1>'-'<h6>' tags

What do I have to change ? Change from code or from theme editor? Thanks!