I can't use the general settings page. It also now showing some other errors, that is: 

OwlError: An error occured in the owl lifecycle (see this Error's "cause" property)
    at handleError (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:916:101)
    at App.handleError (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:1544:29)
    at ComponentNode.initiateRender (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:1006:19)

Caused by: Error: "res.config.settings"."use_mailgateway" field is undefined.
    at Field.parseFieldNode (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:7638:231)
    at http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:8514:865
    at visit (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4219:51)
    at visitChildren (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4218:171)
    at visit (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4219:129)
    at visitChildren (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4218:171)
    at visit (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4219:129)
    at visitChildren (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4218:171)
    at visit (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4219:129)
    at visitChildren (http://localhost:8000/web/assets/6cce691/web.assets_web.min.js:4218:171)