onMounted is called when there is no active component instance to be associated with. Lifecycle inje

这个是说明我们的 onMounted 没有在 setup 里面使用，来看一下我的代码

<script lang="ts">
import { ref, onMounted } from 'vue'
import trimxxx from './trimxxx.vue'
const childRef = ref(null)

onMounted(() => {
  // 访问子组件实例
  // console.log(childRef.value)
  console.log('aaaa')
})
</script>

<template>
  <trimxxx ref="childRef" />
</template>

<style scoped></style>

这是报错

我这里的原因是