Heartbeat notification is not closed after the add-on has been disabled

[Softvision-VladBacia]

[Affected versions]: -Firefox 52.0 and up -Pulse v1.1 RC

[Affected Platforms]: -All Windows -All Mac -All Linux

[Prerequisites]: -Pulse v1.1 RC was previously installed on a clean profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Navigate to a http or https webpage.
3. Open another tab with about:addons -> Extensions screen.
4. Disable and Enable the Pulse add-on and quickly go to the opened page from step 2.
5. After a few seconds the Heartbeat notification is displayed.
6. Disable the Pulse add-on.
7. Click on "X" button from the Heartbeat notification.

[Expected result]:

• Pulse add-on is disabled and also all prefs and UI related to it.

[Actual result]:

• Heartbeat notification is displayed and cannot be closed. The browser needs to be restarted in order to close the Heartbeat notification.

[Notes]:

• In Browser Console the following error is displayed:

TypeError: can't access dead object  (unknown)
TypeError: this.eventCallback is not a function[Learn More]  notification.xml:439:15
	dismiss chrome://global/content/bindings/notification.xml:439:15
	oncommand chrome://browser/content/browser.xul:1:1

• Recording with the issue: