Can’t edit sidebar

I was editing the sidebar, and took something out (the rss feed latest thing) and now all I have left is my about thing. But I wanted to add that back. Or anything for that manner, because there is nothing on the sidebar when I’m not on the main page, as the about thing isn’t shown then.

Now, whenever I put anything on, any character – I get an invalid characters error message on the sidebar. What do I do? How do I edit it again and get back my other sidebar features?