I fixed the issue without version change. So you may download 4.18.5 again and replace class-ure-lib-pro.php file with the latest one.
There is no syntax error apparently at the line 36. But it is raised at PHP versions 5.4 and older. My fault that I did not test the update with earlier PHP versions.
Function is_license_key_editable() is used to show URE’s Settings page.