This is a problem with a plugin or theme. As you're using the core theme it's probably a plugin.
Try removing plugins one by one and see if the message goes away.
You can also debug the problem by following these steps:
- Uncomment line 431 in qa-base.php and reload the page.
- You'll see a bunch of code which is the code where the error came from, copy paste that into a text editor or something with line numbers.
- Look at the lines referenced there (128, 130). That's where the error will be.