The root node of [bug report] tree cannot use insertBefore

[happyfeet1996]

Element UI version

2.15.2

OS/Browsers version

chrome

Vue version

2.6.14

Reproduction Link

https://codepen.io/ziyoung/pen/sdfasd

Steps to reproduce

使用tree的懒加载，第一次加载了十一个子节点，最后节点是more，想使用点击more加载更多节点，使用了inserbefore方法，根结点上的more无法使用，会报错，但是第三级的节点正常

What is Expected?

修复bug

What is actually happening?

修复

[element-bot]

Translation of this issue:

Element UI version

2.15.2

OS/Browsers version

chrome

Vue version

2.6.14

Reproduction Link

https://codepen.io/ziyoung/pen/sdfasd

Steps to reproduce

Lazy loading of tree is used. Eleven child nodes are loaded for the first time, and the last node is more. If you want to use more to load more nodes, the inserbefore method is used. The more on the root node cannot be used, and an error will be reported, but the third level node is normal

What is Expected?

Fix bugs

What is actually happening?

repair