The text style formatting options are wrongly disabled after the "Heading 1" option is double clicked, if the note is empty

[Softvision-MariusComan]

[Affected versions]:

• Firefox 60.0.1 and above

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest version of the "Firefox Notes" add-on (4.1.0dev custom built on 23.05.2018) installed.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Click the "New Note" button from the top part of the sidebar.
3. Click the "Font Size" selector button and double-click the "Heading 1" option.
4. Observe the "Bold", "Italic" and "Strikethrough" buttons.

[Expected result]:

• The text style formatting options are active and actionable.

[Actual result]:

• The text style formatting options are disabled.

[Notes]:

• This issue is reproducible with the TxP version of "Firefox Notes".
• If you will start typing after following the steps, the heading will be wrongly reset to "Body".
• This issue is not reproducible if any other heading option is double clicked instead.
• This issue is also not reproducible if you have content inside the note.
• Attached a screen recording of the issue:

[cedricium]

This is actually reproducible without needing to click on the Heading 1 menu item - if you just double-click right below the editor toolbar (where the formatting options are), you can reproduce this.

Similarly, if you Ctrl/CmdA on an empty note, the same thing will happen. Something to do with selection... plus this issue is reproducible on https://ckeditor.com/ckeditor-5/.