"TypeError: selectedPos is undefined" error is displayed after trying to save a selection using the "Enter" key

[SoftVision-CosminMuntean]

[Affected versions]:

• Nightly 54.0a1 (Build ID: 20170303030202)
• Page Shot 5.2.6242

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• The "xpinstall.signatures.required" is set to "false" and latest Page Shot (5.2.6242) custom build is installed on a new clean profile.

[Steps to reproduce]:

1. Open Firefox with the profile from prerequisites and navigate to any website.
2. Click the Page Shot button from browser toolbar.
3. Perform a selection on the page.
4. Press the "Enter" key and observe the browser behavior.

[Expected result]:

• The selection is successfully saved and you are redirected to the saved shot preview page.

[Actual result]:

• A "TypeError: selectedPos is undefined" error is displayed and the selection is not dismissed.

[Addition Notes:]

• On Page Shot Test Pilot version, after pressing the "Enter" key, the selected area was saved and you were redirected to the saved shot preview page.
• Here is the screen recording with the issue: https://goo.gl/jBnSlb.
• Here is a screen shot with browser console errors: