fix: 解决 Safari 浏览器报错的问题

src/platforms/h5/helpers/get-window-offset.js

@@ -21,10 +21,10 @@ export default function getWindowOffset () {
   }
   const app = getApp()
   if (app) {
-    bottom = app.$children[0].showTabBar ? TABBAR_HEIGHT : 0
+    bottom = app.$children[0] && app.$children[0].showTabBar ? TABBAR_HEIGHT : 0
   }
   return {
     top,
     bottom
   }
-}
+}