Attempting to create a Webhook based Build notification fails with "URL must not be empty" error

Issue Summary

When attempting to add a Webhook Build notification, the screen just sends a "URL must not be empty" error message, rather than saving the notification config.

Steps to Reproduce

1. Edit Build
2. Go to Notifications tab
3. Add Notification
4. Choose the following options:
   Recipient Type: Webhooks
   Webhook name: Build webhook
   URL: any url here

Expected Results

Notification is saved, and notifications sent to webhook

Actual Results

no messages logged in atlassian-bamboo.log or catalina.out

HAR file from recreation will be attached in comments

Experiencing this issue in Safari, Chrome, and Firefox. not tested in IE, or Edge

Workaround

• disable https://marketplace.atlassian.com/apps/1213289/slack-notifications-for-bamboo plugin or
• use Bamboo Specs for Webhook notification configuration