Popup is blank when privacy.userContext.enabled is false

[brizental]

Before submitting a bug report

• [X] I updated to the latest version of Multi-Account Container and tested if I can reproduce the issue
• [X] I searched for existing reports to see if it hasn't already been reported

Step to reproduce

1. Disable privacy.userContext.enabled in about:config
2. Attempt to open the MAC popup

Actual behavior

Popup is blank

Expected behavior

Popup shows something

Additional informations

No response

Provide a copy of Troubleshooting Information page (optional)

No response

[dannycolin]

I think we should prevent the pref to be flipped to false if an addon using the ContextualIdentity is enabled. This would need to be fixed in m-c. I'll try to ask around if there's any objection to this solution.