Use custom default block value instead of None.

wagtail_react_streamfield/widgets.py

@@ -118,7 +118,7 @@ def get_definition(cls, key, block):
             block_definition['className'] = block.meta.classname
         if isinstance(block, FieldBlock):
             block_definition['html'] = block.render_form(
-                block.to_python(None), prefix='field-__ID__')
+                block.get_default(), prefix='field-__ID__')
             title_template = cls.get_title_template(block.name, block)
             if title_template is not None:
                 block_definition['titleTemplate'] = title_template