The strange case of side navigation

[thinkingjimmy]

This is _meta.js of my website:

export default{
  "index": "Introduction",
  "folder1": "folder1",
  "folder2": {
    "title": "folder2",
    "type": "page"
  },
  "about": {
    "title": "About",
    "type": "page"
  },
  "contact": {
    "title": "Contact ↗",
    "type": "page",
    "href": "https://twitter.com/shuding_",
    "newWindow": true
  }
}

when I go to the index page, it looks ok:

but when I click the folder 2 button on nav bar, it looks strange:

I know this will fix the problem. However I don't want to make a homepage:

export default{
  "index": {
    "title": "Home",
    "type": "page"
  },
  "folder1": {
    "title": "folder1",
    "type": "page"
  },
  "folder2": {
    "title": "folder2",
    "type": "page"
  },
  "about": {
    "title": "About",
    "type": "page"
  },
  "contact": {
    "title": "Contact ↗",
    "type": "page",
    "href": "https://twitter.com/shuding_",
    "newWindow": true
  }
}

How do I fix this?