Fix contextual_sidebar null function binding

What does this MR do?

This MR fixes this error that was being thrown in the console on master:

contextual_sidebar.js?bc03:77 Uncaught TypeError: Cannot read property '$innerScroll' of null
    at toggleSidebarOverflow (contextual_sidebar.js?bc03:77)

What are the relevant issue numbers?

Closes #56394 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer