[Experiment] A 500 error message is displayed after clicking the email verification link in a different profile

[ghost]

[Affected versions]:

• Firefox for IOS 27.0 (18428)
• Firefox for Android 68.10.1
• Firefox Release 78.0.1 (Build ID: 20200630195452)
• Microsoft Edge 44.18362.449.0
• Google Chrome 84.0.4147.89

[Affected Platforms]:

• IOS 13.5.1 (iPhone 7+)
• Android 10 (Motorola One Vision)
• Windows 10 x64

[Prerequisites]:

• Have the Firefox browser installed.
• Have 2 new Firefox profiles A and B opened.

[Steps to reproduce]:

1. Open the Firefox browser with the profile A from prerequisites and navigate to the “https://monitor-v2.herokuapp.com/?experimentBranch=vb” page.
2. Sign in to Firefox Monitor using a valid email.
3. Scroll down to the "Want to add another email?" section and add another new email.
4. Switch to the profile B where the Firefox Monitor wasn’t used before and open the verification email received.
5. Click on the “Verify Email” button.
6. Observe the opened page.

[Expected result]:

• The "https://monitor-v2.herokuapp.com/user/verify” page with the “Email Successfully Verified!” message is displayed.

[Actual result]:

• The page is not loaded and the “TypeError: Cannot read property 'treatmentBranch' of undefined” error is displayed.

[Notes]:

• This issue is reproducible for both experiment branches.
• This issue is not reproducible if the email verification is performed in the same profile.
• Attached is the screenshot of the error.

[ghost]

I have verified this issue and is no longer reproducible.

• The "https://monitor-v2.herokuapp.com/user/verify” page with the “Email Successfully Verified!” message is displayed after clicking the email verification link in a different profile.
• Tested using Firefox Nightly 80.0a1(Linux Mint 19.2, Windows 10), Firefox Beta 79.0(Windows 10, Linux Mint 19.2), Firefox Release 78.0.2(Windows 10), Firefox for Android 68.10.1 (Android 10), Firefox for iOS 27.0 (iOS 13.5.1).