Type-safe result of `validate`

[Luncher]

code

    this.configState.validate().then(res => {
      if (res.hasError) {
        this.props.toasterStore.error(res.error)
                                      // ^^^^^^-Type 'undefined' is not assignable to type 'string'.
        return
      }
      //...
    })

Expected behavior

the type of re.error should be inferred correctly