The "Undo" option is wrongly displayed after the "TAAR" shield study is removed

[Softvision-MariusComan]

[Affected versions]:

• Firefox 61.0

[Affected Platforms]:

• All Windows
• All Linux
• All Mac

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "TAAR v3" add-on (downloaded from https://bugzilla.mozilla.org/show_bug.cgi?id=1469546) installed.
• Create the following prefs in the "about:config" page:
  • "shieldStudy.logLevel" as string and set it to "All";
  • "extensions.taarexpv3_shield_mozilla_org.test.variationName" as string and set it to "control", "intervention-a" or "intervention-b"

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Navigate to "about:addons>Extensions" and click the "TAAR v3" add-on.
3. Click the "Remove" button and observe the behavior.

[Expected result]:

• The "TAAR" shield study is irreversibly removed.

[Actual result]:

• The "Undo" button is displayed.

[Notes]:

• After the "Undo" button is clicked, the "install" and "enter" pings are sent again.
• Attached a screen recording of the issue: