backoffice: don't crash previewing fields that fail to render (#46099)

wcs/admin/forms.py

@@ -876,7 +876,13 @@ class FormDefPage(Directory):
         for i, field in enumerate(self.formdef.fields):
             field.id = i
             if hasattr(field, str('add_to_form')):
-                field.add_to_form(form)
+                try:
+                    field.add_to_form(form)
+                except Exception as e:
+                    form.widgets.append(HtmlWidget(htmltext(
+                        '<div class="errornotice"><p>%s (%s)</p></div>') % (
+                            _('Error previewing field.'),
+                            e)))
             else:
                 if field.key == 'page':
                     if on_page: