Value of type 'StorageMetadata' has no member 'downloadURL'

In File.swift l.130 `return self.metadata?.downloadURL()` , the above error has occurred.

```
   /// DownloadURL
    public var downloadURL: URL? {
        if let url: URL = self._downloadURL {
            return url
        }
        return self.metadata?.downloadURL()
    }
```

What should I change this code?
