Fix "Saved groups" and "Sessions" not expanding

Fix "Saved groups" and "Sessions" not expanding in Opera 25 Developer (on Linux at least)

According to http://javascript.about.com/library/bldom12.htm, insertBefore requires 2 parameters, seting the second parameter to null seems to fix TabHamster for me.
Merge pull request #14 from richarson/opera25dev

Fix "Saved groups" and "Sessions" not expanding