fix: problem of integration with vuetify (#40)

If there was installed this package and vuetify there was errors described in issue

> meteor/meteor#10093 (comment)

Errors was induced by trying to iterate over `this._trackerHandles` that has value equal null.
Checking if `this._trackerHandles` is array before iteration we can fix this error.

Author: gustawdaniel

src/index.js

@@ -253,13 +253,15 @@ export default {
 
         $stopMeteor () {
           // Stop all reactivity when view is destroyed.
-          this._trackerHandles.forEach((tracker) => {
-            try {
-              tracker.stop()
-            } catch (e) {
-              if (Meteor.isDevelopment) console.error(e, tracker)
-            }
-          })
+          if(Array.isArray(this._trackerHandles)) { // sometimes this._trackerHandles is null and we want skip this step
+            this._trackerHandles.forEach((tracker) => {
+              try {
+                tracker.stop()
+              } catch (e) {
+                if (Meteor.isDevelopment) console.error(e, tracker)
+              }
+            })
+          }
           this._trackerHandles = null
           this._meteorActive = false
         },