Changes proposed in this Pull Request
Fixed error after click to button "Show URLs where used"

Testing instructions

• Go to urlslab plugin "Lazy Loading"->"YouTube Videos" and click
  "Show URLs where used" button and check console to error and should
  to show popup window

Checklist

• My code follows the style guidelines of this project
• My commits follow the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Close QualityUnit/urlslab-issues#1546