The goblin should not have been on the island until after you talked to the tree.
Do you have a save before then you can reload?
Did you try exiting the game and reloading? How about going to Broken Valley and returning, to see if that would reset something. You could also try talking to the statues again in the primordial cave.
Try emailing support@larian.com; they may be able to reset the status of the tree, or something.
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).