The "Page saved as" pop-up is wrongly displayed on page recent version, if the page was previously saved

[SoftVision-CosminMuntean]

[Affected versions]:

• Nightly 54.0a1
• NoMore404s 1.8.1

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Latest NoMore404s add-on (v1.8.1) was previously installed on a new profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite.
2. Navigate to any website (eg. https://www.amazon.com/).
3. Click the "NoMore404s" button and choose the "Save Page Now" option.
4. After the page is saved, closed the tab in order to return to the opened website in step 2.
5. Click the "NoMore404s" button and choose the "Recent Version" option.
6. Observe the page recent version.

[Expected result]:

• The page recent version is successfully loaded without any additional pop-ups opened.

[Actual result]:

• The "Page saved as" pop-up is wrongly displayed on the page.

[Notes]:

• Here is a screen recording with the issue: https://goo.gl/YcYn2Z.