Swift unable to retrieve child node firebase database

Question

My database looks like this

category
   subCat1
      item1
         someData: somedata
         storage: storageref
      item2
         someData: somedata
         storage: storageref
      item3
         someData: somedata
         storage: storageref
   subCat2
      item1
         someData: somedata
         storage: storageref
      item2
         someData: somedata
         storage: storageref
      item3
         someData: somedata
         storage: storageref

I would like to retrieve the value of the "storage" key in a specific subcategory node. I've tried tapping into one specific subcategory and retrieve to storage key with the following code:

let ref = Database.database().reference().child("category").child("subCat1")
    ref.observeSingleEvent(of: .value) { (snapshot) in

        for item in snapshot.children {
            let snap = item as! DataSnapshot
            let imageSnap = snap.childSnapshot(forPath: "storage")
            let dict = imageSnap.value as! [String: Any]
            let url = dict["storage"] as! String
            print(url)
        }

This code crashes the app on the line let dict = imageSnap.value as! [String: Any] with the error code Could not cast value of type '__NSCFString' (0x10632a168) to 'NSDictionary' (0x10632b1a8)

I've tried printing snapshot, snapshot.child, snapshot.value etc. None of these give me what I'm looking for, they either do not work, or give me one large dictionary containing subCat1, all items, someData and storage.

Update:

with the help of Sh_Khan's answer below, I'm able to print the storage strings for each item in the node, however my for in loop causes the all the strings to be printed 3 times each so I get a total of 3 x 3 urls when I only want 3 (one for each item)

Answer 1

Replace

let dict = imageSnap.value as! [String: Any]
let url = dict["storage"] as! String

with

if let url = imageSnap.value as? String { } 

Answer 2

Above Sh_Khan answer is correct, just try to unwrap item value using if let so you don't get crash. Your updated code below:

    let ref = Database.database().reference().child("category").child("subCat1")
        ref.observeSingleEvent(of: .value) { (snapshot) in

            for item in snapshot.children {
                if let snap = item as? DataSnapshot {

                 let imageSnap = snap.childSnapshot(forPath: "storage")
                 let url = imageSnap.value as? String

                 print(url)

                }
            }
   }