Internal error on calling /mine [GET]

I followed the instruction. Tried http://localhost:80/mine in postman.

```
> docker run --rm -p 80:5000 blockchain
* Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

[2020-03-04 19:22:19,375] ERROR in app: Exception on /mine [GET]
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "blockchain.py", line 229, in mine
return jsonify(response), 200
File "/usr/local/lib/python3.6/site-packages/flask/json.py", line 251, in jsonify
if current_app.config['JSONIFY_PRETTYPRINT_REGULAR'] and not request.is_xhr:
File "/usr/local/lib/python3.6/site-packages/werkzeug/local.py", line 347, in __getattr__
return getattr(self._get_current_object(), name)
AttributeError: 'Request' object has no attribute 'is_xhr'
172.17.0.1 - - [04/Mar/2020 19:22:19] "GET /mine HTTP/1.1" 500 -
```

System Windows 10, Docker version 2.2.0.3.

Btw. The strange thing is that it shows it's running on 0.0.0.0 instead of 127.0.0.1.