The note menu is wrongly dismissed if it's clicked and no option is focused

[Softvision-MariusComan]

[Affected versions]:

• Firefox 60.0.2

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Firefox Notes" add-on (4.2.0dev custom built on 20.06.2018) installed.
• Have at least a note in the sidebar.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Open any note and click the menu button from the right side of the note's title.
3. Click on the gray separator line between the "New Note" option and the "Export as HTML..." options.
4. Observe the behavior.

[Expected result]:

• Nothing happens, the menu is still displayed.

[Actual result]:

• The menu is dismissed.

[Additional Notes]:

• Attached a screen recording of the issue:

[vladikoff]

We have a PR for this..