def extend(self, iterable):
for item in iterable:<EOL><INDENT>if not isinstance(item, str):<EOL><INDENT>self.append(item)<EOL><DEDENT><DEDENT>return None<EOL>
django.forms.forms.BaseForm._html_output() extends non_field_errors
with a string error for each hidden field. The string format is incompatible
with djng TupleErrorList of SafeTuples causing an exception in as_ul.
Instead we discard extends containing strings here and add errors in non_field_errors
for hidden fields in NgFormBaseMixin