原因：将扁平数据转化为树形数据时，给每个项都添加了 children

export const transList2Tree = (list, rootPid) => {
  const result = []
  list.forEach(item => {
    if (item.pid === rootPid) {
      const children = transList2Tree(list, item.id)
      item.children = children
      result.push(item)
    }
  })
  return result
}

修改代码：

export const transList2Tree = (list, rootPid) => {
  const result = []
  list.forEach(item => {
    if (item.pid === rootPid) {
      const children = transList2Tree(list, item.id)
      // 只有 children不为空时，才继续向下找 children 的数据
      if (children.length > 0) {
        item.children = children
      }
      result.push(item)
    }
  })
  return result
}