run () {
    if (this.active) {
      const value = this.get() 
      if (
        value !== this.value ||
        // Deep watchers and watchers on Object/Arrays should fire even
        // when the value is the same, because the value may
        // have mutated.
        isObject(value) ||
        this.deep
      ) {
        // set new value
        const oldValue = this.value
        this.value = value
        if (this.user) {
          try {
            this.cb.call(this.vm, value, oldValue)
          } catch (e) {
            handleError(e, this.vm, `callback for watcher "${this.expression}"`)
          }
        } else {
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }

执行到 watcher.run 函数中，会先执行 this.get()，即会执行 updateComponent 函数，导致页面重新渲染，之后才进入到 user watcher 的回调函数中，导致无限添加 watcher 到队列中，那么为什么页面没有将 msg 渲染成随机数？而是先报错？

我点击 toggle 后，随机数就显示了