TreeGraph在微信小程序上如果数据层级超过2似乎会报错，无法绘制

我是用的uniapp来开发微信小程序

当我给graph传入这样的数据时，可以成功绘制
```javascript
graph = new F6.TreeGraph({/*...*/})
// ...
// rootNode, deviceNode是我自己定义的节点类型，用的F6.registerNode()
graph.data({
id: 'root',
type: 'rootNode',
children: [
{
id: '111',
type: 'deviceNode',
},{
id: '222',
type: 'deviceNode',
}
]
})
```

当我给graph传入这样的数据时，会出现TypeError: Cannot read property 'index' of null，无法绘制树图
```javascript
graph = new F6.TreeGraph({/*...*/})
// ...
graph.data({
id: 'root',
type: 'rootNode',
children: [
{
id: '111',
type: 'deviceNode',
children: [
{
id: '333',
type: 'deviceNode'
}
]
},{
id: '222',
type: 'deviceNode',
}
]
})
```
请问是不是我哪里写的有问题

Start by reproducing the nested-children example with F6.TreeGraph, custom rootNode/deviceNode types, and uni-app on a WeChat mini program. Compare it with the working two-level example and trace the TypeError about reading index from null. Done means a tree with a child at the third level renders without the error.

Written by the indexing model from the issue text.