Make sure offset and start called safety.

Author: tangbc

examples/build/finite.js

@@ -282,7 +282,10 @@
 
             // set manual scroll top.
             setScrollTop: function (scrollTop) {
-                this.$refs.vsl.scrollTop = scrollTop
+                var vsl = this.$refs.vsl
+                if (vsl) {
+                    vsl.scrollTop = scrollTop
+                }
             },
 
             // filter the shown items base on `start` and `end`.

examples/build/infinite.js

@@ -1,6 +1,6 @@
 {
   "name": "vue-virtual-scroll-list",
-  "version": "1.2.5",
+  "version": "1.2.6",
   "description": "A vue (2.x) component that support big data list with high scroll performance.",
   "main": "index.js",
   "files": [