[erp5_hal_json_style] FormBox should not crash if form_id is not defined

bt5/erp5_hal_json_style/SkinTemplateItem/portal_skins/erp5_hal_json_style/ERP5Document_getHateoas.py

@@ -774,7 +774,12 @@ def renderField(traversed_document, field, form, value=MARKER, meta_type=None, k
         field=field, REQUEST=REQUEST)
       embedded_document['_debug'] = "Different context"
     # get embedded form definition
-    embedded_form = getattr(formbox_context, field.get_value('formbox_target_id'))
+    embedded_form_id = field.get_value('formbox_target_id')
+    embedded_form = None
+    if embedded_form_id:
+      embedded_form = getattr(formbox_context, embedded_form_id, None)
+    if embedded_form is None:
+      return
     # renderForm mutates `embedded_document` therefor no return/assignment
     renderForm(formbox_context, embedded_form, embedded_document, key_prefix=key)
     # fix editability which is hard-coded to 0 in `renderForm` implementation