"Note summary" wrongly contains characters from the first paragraph if it has more than 250 characters

[Softvision-AndreeaCupsa]

[Affected versions]:

• Firefox 59.0.3 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

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

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Create a note with 1 paragraph (having 251 characters).
3. Go back to preview note list and observe the list.

[Expected result]:

• The "Note summary" is empty

[Actual result]:

• The "Note summary" contains the 251st character from the first paragraph.

[Notes]:

• This issue is reproducible regardless of how many characters are over 250.
• If there is a second paragraph and space left in "Note summary", it will only be displayed after the first paragraph runs out of characters.
• This issue is not reproducible on Android with "Notes"(notes-1_0-qa-v2055) app installed.
• Attached a screen recording of the issue: