Cannot find type 'SFSafariExtensionHandler' in scope

Hello,

I am creating a Safari App Extension. I downloaded the Sea Creator example project.

I want to send messages between the extension and javascript. According to the documentation provided by Apple I should be able to do it with the following function:

override func messageReceived(withName messageName: String, from page: SFSafariPage, userInfo: [String : AnyObject]!) {
    page.dispatchMessageToScript(withName: "simpleMessage", userInfo: nil)
    page.dispatchMessageToScript(withName: "complexMessage", userInfo: ["myKey": "myValue"])
}

In the Sea Creator project I modified the SafariWebExtensionHandler to

import SafariServices

class SafariWebExtensionHandler: SFSafariExtensionHandler {
  override func messageReceived(withName messageName: String, from page: SFSafariPage, userInfo: [String : AnyObject]!) {
    page.dispatchMessageToScript(withName: "simpleMessage", userInfo: nil)
    page.dispatchMessageToScript(withName: "complexMessage", userInfo: ["myKey": "myValue"])
  }
}

But I am getting a buildtime error: Cannot find type 'SFSafariExtensionHandler' in scope

I am running Xcode 13.0 beta (13A5154h) on Mac OS version 11.4

What I find weird is that I downloaded a random project from github and it doesn't throw me the same error. I am wondering if there is some additional configuration I am missing.