Mouse out over legend should not cause errors on the console (#9274)

Backports PR #9263

**Commit 1:**
Mouse out over legend should not cause errors to the console

* Original sha: e688065
* Authored by Thomas Neirynck <thomas@elastic.co> on 2016-11-29T21:44:03Z

src/ui/public/visualize/visualize_legend.js

@@ -40,7 +40,10 @@ uiModules.get('kibana')
       $scope.unhighlight = function (event) {
         let el = event.currentTarget;
         let handler = $scope.renderbot.vislibVis.handler;
-        if (!handler) return;
+        //there is no guarantee that a Chart will set the unhighlight-function on its handler
+        if (!handler || typeof handler.unHighlight !== 'function') {
+          return;
+        }
         handler.unHighlight.call(el, handler.el);
       };