Screenshots works in Private Browsing Mode, if it's used from Nightly "Page Actions" menu

[SoftVision-CosminMuntean]

[Notes]:

• If Screenshots is used from context menu, a "Screenshots is disabled in Private Browsing Mode" message is displayed. But, Screenshots works if it's used from "Page Actions".
• This behavior can be confused for the users, because Screenshots works from "Page Actions" menu, but is not working from context menu.

[Affected versions]:

• Firefox 57.0a1
• Screenshots 16.1.0
• Screenshots 19.0.0

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Start the latest Nightly browser and open a Private Window.
2. Navigate to any website and click the "Page Actions" button.
3. Click the Screenshots button and observe the behavior.

[Expected result]:

• A "Screenshots is disabled in Private Browsing Mode" message is displayed.

[Actual results]:

• The Screenshots overlay covers the entire page.

[Additional Notes]:

• We decided to disable Screenshots in private windows in issue Disable in private browsing mode #3120
• Here is a screen recording with the issue:
  

[SoftVision-CosminMuntean]

I have retested this issue on Screenshots dev server installed from: https://screenshots.stage.mozaws.net/homepage/install-test-local.html.

The Screenshots no longer works on Private Window, but the "Screenshots is disabled in Private Browsing Mode" error message is not displayed if you click the Screenshots button in Private Window.

I think the error message should be displayed after clicking the "Screenshots" button, because the user can be confused why nothing happens.
@ianb should I log a different issue for this?

[SoftVision-CosminMuntean]

I just retest this issue using latest Nightly 58.0a1 build using a new clean profile with Screenshots installed from: https://screenshots.dev.mozaws.net/homepage/install-test-local.html.
The issue is no longer reproducible and the "Screenshots is disabled in Private Browsing Mode" error message is correctly displayed.