The error is an E_NOTICE. Those kinds of errors are generally not serious at all, except some cases when the evaluation to NULL causes the script to enter an unexpected code block through a conditional that expects a populated offset.
The remote version check, which queries our server to find out if a new version is available to download and displays that information on the same page, only occurs on the wiki admin index page. This error is a result of that version check failing for some reason. It will not cause problems in any part of the wiki, because it is not used or referred to anywhere else. While it is possible that you might see this error regularly, it would only be when visiting that admin page.