The "Note" toolbar button no longer works after a new normal window is opened from a private one

[Softvision-MariusComan]

[Affected versions]:

• Firefox 61.0.1 and above

[Affected Platforms]:

• All Windows
• All Linux
• All Mac

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Firefox Notes" add-on (4.2.0dev), custom built on 23.07.2018, installed.
• The "Notes" sidebar is opened.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Open a new private window.
3. Open a new normal window.
4. Click the "Notes" toolbar button and observe the behavior.

[Expected result]:

• The "Notes" sidebar is opened.

[Actual result]:

• Nothing happens.

[Notes]:

• This issue is not reproducible if the "Show sidebars" toolbar button is clicked instead.
• This issue is no longer reproducible after the "Notes" sidebar is opened.
• Attached a screen recording of the issue: