Compatibility with Admin Toolbar 3.6.0

Quick follow-up on this issue:

Created the initial merge request MR !37 above at #2 which :

Converts the __construct method to create and all the existing code or logic was kept pretty much "as-is".

After testing the changes locally, the error disappeared and the expected menu items seemed to display properly under Content, for example: this patch should fix the fatal error.👌

Moving issue to Needs review for now, as an attempt to attract more attention, reviews, testing and reporting feedback.

Feel free to let us know if you have any questions on this issue or the changes suggested in the merge request, we would certainly be glad to help.
Thanks in advance!

Thanks a lot, @dydave! Your patch for admin_toolbar_content works very well here!

Thanks for the fix. I merged it.