Well, do you want to get rid of the problem, or just hide it?
If you just want to hide it, put this in a file somewhere where you know it will get executed every page load:
What this does it suppresses all error messages below critical. So it will suppress warnings and notices.
To get rid of the error (if you do not know how to fix it), go to the line of the code and copy and paste it into here so we can look at it.