Replace content_json with content as per https://docs.wagtail.org/en/…

…stable/releases/3.0.html#replaced-content-json-textfield-with-content-jsonfield-in-pagerevision

core/signals.py

@@ -100,7 +100,7 @@ def pre_page_revision_save(sender, instance, **kwargs):
                     #  Replace content field
                     setattr(mirror_page, field.name, clean_field)
                     #  To json again
-                    instance.content_json = mirror_page.to_json()
+                    instance.content = mirror_page.to_json()
 
 
 @receiver(post_save)