Event Definition : can't convert undefined to object

When a Notification is set on an Event Definition, an error "Something went wrong" is displayed when viewing the Event Definition.

## Expected Behavior
When clicking on the name of an Event Definition, a page is displaying the current configuration of the Event Definition.

## Current Behavior
On the Event Definition that use a specific Notification (Legacy callback, for Mattermost), Graylog throws an error when clicking on the Event Definition title, thus preventing the display of its current configuration and preventing the alert to be correctly published to Mattermost

```
can't convert undefined to object

Stack Trace:

value@https://graylog.acme.com/assets/1b3f1b67-51.3433c0ea0395e9c3a16a.js:1:2122
Hi@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:293781
Ri@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:293576
Ls@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:329190
Mc@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:320633
mc@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:320556
sc@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:317586
Za/<@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:269227
434/exports.unstable_runWithPriority@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:780503
Ua@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:268936
Za@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:269174
Va@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:269107
nc@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:314371
yi@https://graylog.acme.com/assets/vendor.594b2a39cb22b445205e.js:2:286185
E/