The "Manage Snoozed Tabs" empty panel is displayed for a milisecond after clicking the "Undo" button

[SoftVision-CarmenFat]

[Affected versions]:

• Release 58.0 and up
• Snooze Tabs 1.0.21

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest Snooze Tabs add-on version (https://669-15314951-gh.circle-artifacts.com/0/tmp/circle-artifacts.kZdit8p/addon.xpi) loaded as a temporary add-on.
• Have one snoozed tab in the "Manage Snoozed Tabs" panel.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite and navigate to a website.
2. Click the "Snooze Tabs" button from the toolbar.
3. Click the "Manage Snoozed Tabs" button.
4. Click on the "Delete" button.
5. Click the "Undo" button and observe the panel's UI.

[Expected result]:

• The panel's UI doesn't change.

[Actual result]:

• The "Manage Snoozed Tabs" empty panel is displayed for a milisecond.

[Notes]:

• This is not reproducible if having more than one snoozed items in the panel.
• Attached a screen recording of the issue: https://goo.gl/Bb8NeR.