FirebaseStorage 画像保存時のurlを取得について

FrebaseStorageに画像を保存する際にfirebase databaseに保存した画像のURLを保存したいのですがエラーが出てしまいます

###前提

Swift
1let metadata = FIRStorageMetadata()
2        metadata.contentType = "image/jpg"
3        
4        let image = self.PostFilterImage
5        if let data = UIImageJPEGRepresentation(image, 0.8) {
6            let imagePath = FIRAuth.auth()!.currentUser!.uid + "/\(Int(NSDate.timeIntervalSinceReferenceDate() * 1000)).jpg"
7  
8            let photoRef = storageRef.child(filePath).child(imagePath)
9            
10            
11            photoRef.putData(data, metadata: metadata, completion: { (metaData, error) in
12                
13                if let error = error {
14                    print(error.localizedDescription)
15                    return
16                    
17                } else {      
18                    
19                    let fileUrl = metadata.downloadURL()!.absoluteString  エラー部分                
20
21                    let MessageData = ["user": (FIRAuth.auth()?.currentUser?.uid)!,"content": self.postText.text!, "date": FIRServerValue.timestamp(), "fileUrl": fileUrl]
22                    
23                    self.databaseRef.child((FIRAuth.auth()?.currentUser?.uid)!).childByAutoId().setValue(MessageData)
24                    
25                }
26                
27            })
28        }
29