IPv6 Crash Firebase Swift - swift

Question

I have a Swift app using Firebase as backend.
It works perfectly fine on all simulators and my own iOS devices, but when I submitted it to Apple for review, it crashed specifically when the reviewer tapped on a UIButton that presents a UIAlertController containing my content reporting mechanism:
#IBAction func moreActions(_ sender: AnyObject) {
// 1
let optionMenu = UIAlertController(title: nil, message: "Choose Action", preferredStyle: .actionSheet)
// 2
let reportPost = UIAlertAction(title: "Report this post", style: .default, handler: {
(alert: UIAlertAction!) -> Void in
self.report()
})
// 3
let cancelAction = UIAlertAction(title: "Cancel", style: .cancel, handler: {
(alert: UIAlertAction!) -> Void in
print("Cancelled")
})
// 4
optionMenu.addAction(reportPost)
optionMenu.addAction(cancelAction)
self.present(optionMenu, animated: true, completion: nil)
}
func report() {
let messageView = MessageView.viewFromNib(layout: .CardView)
var config = SwiftMessages.Config()
config.dimMode = .gray(interactive: true)
messageView.configureTheme(.info)
messageView.button?.isHidden = true
messageView.configureContent(title: "Thank you", body: "We have received your report, and will soon make all necessary actions.")
SwiftMessages.show(config: config, view: messageView)
ref = FIRDatabase.database().reference()
ref.child("reportedPost").observeSingleEvent(of: .value, with: {(snapshot) in
if snapshot.hasChild(self.passedPurchaseKey) {
print("this post has been reported before")
let targetKey = self.passedPurchaseKey
let value = snapshot.childSnapshot(forPath: targetKey!).value as! Int
let newValue = value + 1
self.ref.child("reportedPost").updateChildValues([targetKey!: newValue])
}else{
print("this post has not been reported before")
let targetKey = self.passedPurchaseKey
self.ref.child("reportedPost").updateChildValues([targetKey!: 1])
}
})
}
The reviewer suggested that the crash might due to IPv6 incompatibility. I cannot agree with them since the networking logics above are very similar to the rest of my App, which did not lead to any crashes. Thus I suspect that the crash was due to something else.
Yet I could not be sure because I do not have the required hardwares to test my app under IPv6 environment.
Thanks in advance for your kind help!
//edit 1: crash log added
crashlog_1
crashlog_2