Crash lg - Undefined is not JSON serializable

[takenek]

123.123.123.123 - - [14/Aug/2017 15:00:29] "GET /robots.txt HTTP/1.1" 500 -
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1997, in call
return self.wsgi_app(environ, start_response)
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1985, in wsgi_app
response = self.handle_exception(e)
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1540, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1512, in handle_user_exception
return self.handle_http_exception(e)
File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1471, in handle_http_exception
return handler(e)
File "/opt/bird-lg-master/lg.py", line 200, in page_not_found
return render_template('error.html', warnings=["The requested URL was not found on the server."]), 404
File "/usr/lib/python2.7/dist-packages/flask/templating.py", line 134, in render_template
context, ctx.app)
File "/usr/lib/python2.7/dist-packages/flask/templating.py", line 116, in _render
rv = template.render(context)
File "/usr/lib/python2.7/dist-packages/jinja2/environment.py", line 989, in render
return self.environment.handle_exception(exc_info, True)
File "/usr/lib/python2.7/dist-packages/jinja2/environment.py", line 754, in handle_exception
reraise(exc_type, exc_value, tb)
File "/opt/bird-lg-master/templates/error.html", line 1, in top-level template code
{% extends "layout.html" %}
File "/opt/bird-lg-master/templates/layout.html", line 125, in top-level template code
history_query = {{session.history|tojson|safe}};
File "/usr/lib/python2.7/dist-packages/flask/json.py", line 269, in tojson_filter
return Markup(htmlsafe_dumps(obj, **kwargs))
File "/usr/lib/python2.7/dist-packages/flask/json.py", line 183, in htmlsafe_dumps
rv = dumps(obj, **kwargs)
File "/usr/lib/python2.7/dist-packages/flask/json.py", line 123, in dumps
rv = _json.dumps(obj, **kwargs)
File "/usr/lib/python2.7/dist-packages/simplejson/init.py", line 397, in dumps
**kw).encode(obj)
File "/usr/lib/python2.7/dist-packages/simplejson/encoder.py", line 291, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/usr/lib/python2.7/dist-packages/simplejson/encoder.py", line 373, in iterencode
return _iterencode(o, 0)
File "/usr/lib/python2.7/dist-packages/flask/json.py", line 80, in default
return _json.JSONEncoder.default(self, o)
File "/usr/lib/python2.7/dist-packages/simplejson/encoder.py", line 268, in default
raise TypeError(repr(o) + " is not JSON serializable")
TypeError: Undefined is not JSON serializable