'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'

def extend(self, iterable):

    for item in iterable:
        if (not isinstance(item, str)):
            self.append(item)
    return None
