fix(tooltip): do nothing if $scope doesn't exist

- Do nothing on hide when `$scope` is already removed

Closes angular-ui#4346
Fixes angular-ui#3347

Author: btrauma8

Diff for: src/tooltip/tooltip.js

@@ -234,6 +234,10 @@ angular.module('ui.bootstrap.tooltip', ['ui.bootstrap.position'])
 
             // Hide the tooltip popup element.
             function hide() {
+              if (!ttScope) {
+                return;
+              }
+
               // First things first: we don't show it anymore.
               ttScope.isOpen = false;
               if (isOpenExp) {