Actually reinstalling didn't work for me. There is a config.xml file in "The Divinity Engine" folder you can edit to fix this. Note this is a separate folder than that of the actual game "Divinity - Original Sin".

You can use notepad to to edit the config file. Can copy and paste the lines below if you installed steam and the game to the default directory.

<EditorDataPath>C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Data\Editor\</EditorDataPath>
<DataPath>C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Data\</DataPath>