fix: ensure preview is object with getStorybook() for storybook@3

Author: jholt1

src/server/getStorybook.js

@@ -5,5 +5,5 @@ export default async (browser, iframeUrl) => {
     waitUntil: "networkidle2"
   });
 
-  return page.evaluate("(typeof preview !== 'undefined') ? preview.getStorybook() : __STORYBOOK_CLIENT_API__.getStorybook()");
+  return page.evaluate("(typeof preview === 'object' && typeof preview.getStorybook !== 'undefined') ? preview.getStorybook() : __STORYBOOK_CLIENT_API__.getStorybook()");
 };