BUG: Prevents a warning being thrown when reordering.

Sorry mate, I have been away for a couple of weeks and missed your message on the first pull request for this and it was closed before I could respond.

I was getting the following in the black box at the top right of screen and a broken page in the admin: Warning at line 67 of MYPATH/website/gridfieldextensions/code/GridFieldOrderableRows.php. 

Adding the isset check made the error messages go away and made things a lot smoother when creating content.

Any chance this could be merged?