dispatch_group_leave error while adding new child - ios

Question

I am trying to add a new child to my node in firebase using this code:
#IBAction func likeButtonOnTouch(_ sender: Any) {
if ViewController.usersUid.count > 0 {
self.update()
}
}
func update() {
let child1: String = (FIRAuth.auth()?.currentUser?.uid)!
let ref = FIRDatabase.database().reference().child(child1).child("following")
let data1: [String:String] = [ViewController.usersUid[self.currentUser]: "true"]
ref.setValue(data1)
}
But when I press the button, my app crashes with this error:
thread 1 exc_bad_instruction (code=1 subcode=0x100ef5c78)
on this line:
ref.setValue(data1)
I have no idea what this means and how to fix it. The most interesting thing that the value has successfully added to my DB (!!!), but app crashes. Please give me an advice.
P.S. crash report:
* thread #1: tid = 0xb503, 0x0000000100f11c78 libdispatch.dylib`dispatch_group_leave + 76, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x100f11c78)
frame #0: 0x0000000100f11c78 libdispatch.dylib`dispatch_group_leave + 76
frame #1: 0x00000001000a36f8 Elite Club`thunk + 68 at ViewController.swift:0
frame #2: 0x00000001001bf8a4 Elite Club`__43-[FValueEventRegistration fireEvent:queue:]_block_invoke.57((null)=<unavailable>) + 88 at FValueEventRegistration.m:60 [opt]
frame #3: 0x0000000100f0d258 libdispatch.dylib`_dispatch_call_block_and_release + 24
frame #4: 0x0000000100f0d218 libdispatch.dylib`_dispatch_client_callout + 16
frame #5: 0x0000000100f12280 libdispatch.dylib`_dispatch_main_queue_callback_4CF + 1200
frame #6: 0x000000019376e810 CoreFoundation`__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
frame #7: 0x000000019376c3fc CoreFoundation`__CFRunLoopRun + 1660
frame #8: 0x000000019369a2b8 CoreFoundation`CFRunLoopRunSpecific + 444
frame #9: 0x000000019514e198 GraphicsServices`GSEventRunModal + 180
frame #10: 0x00000001996e17fc UIKit`-[UIApplication _run] + 684
frame #11: 0x00000001996dc534 UIKit`UIApplicationMain + 208
frame #12: 0x00000001000c04b8 Elite Club`main + 140 at AppDelegate.swift:15
frame #13: 0x000000019267d5b8 libdyld.dylib`start + 4

Answer 1

if ViewController.usersUid[self.currentUser] does not exist please crash
Could you Try this way :
let data1: [String:String] = [(ViewController.usersUid[self.currentUser] ?? "") : "true"]
ref.setValue(data1) { (error, ref) -> Void in
}
more cleaner way :
if let currentuser = ViewController.usersUid[self.currentUser] {
let data1: [String:String] = [currentuser : "true"]
ref.setValue(data1) { (error, ref) -> Void in
}
}

Answer 2

After a day of working, I've figured out. First, you should sign out, and then sign in one more time. Here is my final code:
func update() {
do {
let ref = try FIRAuth.auth()?.signOut()
}
catch {}
let preferences = UserDefaults.standard
FIRAuth.auth()?.signIn(withEmail: preferences.object(forKey: "userName") as! String, password: preferences.object(forKey: "userPassword") as! String) { (user, error) in
if error == nil {
LoggingIn.userUID = (user?.uid)!
print(user?.uid ?? "not found")
print("You have successfully logged in")
let child1: String = (FIRAuth.auth()?.currentUser?.uid)!
let ref = FIRDatabase.database().reference().child(child1).child("following")
let data1: [String:String] = [ViewController.usersUid[self.currentUser] : "true"]
ref.setValue(data1) { (error, ref) -> Void in
}
} else {
}
}
}