Ensure that draftail contentstate is never empty

wagtail/admin/rich_text/converters/contentstate.py

@@ -7,6 +7,7 @@ from draftjs_exporter.defaults import render_children
 from draftjs_exporter.dom import DOM
 from draftjs_exporter.html import HTML as HTMLExporter
 
+from wagtail.admin.rich_text.converters.contentstate_models import Block
 from wagtail.admin.rich_text.converters.html_to_contentstate import HtmlToContentStateHandler
 from wagtail.core.rich_text import features as feature_registry
 
@@ -87,6 +88,11 @@ class ContentstateConverter():
     def from_database_format(self, html):
         self.html_to_contentstate_handler.reset()
         self.html_to_contentstate_handler.feed(html)
+
+        if not self.html_to_contentstate_handler.contentstate.blocks:
+            # add an empty paragraph block to make contentstate valid
+            self.html_to_contentstate_handler.add_block(Block('unstyled', depth=0))
+
         return self.html_to_contentstate_handler.contentstate.as_json(indent=4, separators=(',', ': '))
 
     def to_database_format(self, contentstate_json):

wagtail/admin/rich_text/editors/draftail/__init__.py

@@ -35,9 +35,9 @@ class DraftailRichTextArea(WidgetWithScript, widgets.HiddenInput):
 
     def render(self, name, value, attrs=None):
         if value is None:
-            translated_value = None
-        else:
-            translated_value = self.converter.from_database_format(value)
+            value = ''
+
+        translated_value = self.converter.from_database_format(value)
         return super().render(name, translated_value, attrs)
 
     def render_js_init(self, id_, name, value):