Firestore Only add if doesn't exists

gistfile1.txt

class AddDeviceFragment : Fragment() {

    private var _binding: FragmentAddDeviceBinding? = null
    private val binding get() = _binding!!

    //firestore instance
    private lateinit var firestore: FirebaseFirestore

    //auth instance
    private lateinit var auth: FirebaseAuth

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        _binding = FragmentAddDeviceBinding.inflate(layoutInflater)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        //Initialising firestore and auth
        firestore = FirebaseFirestore.getInstance()
        auth = FirebaseAuth.getInstance()

        binding.btnAddDevice.setOnClickListener {
            val device = binding.etAddDevice.text.toString()
            saveInFirestore(device)
        }
    }

    // To fix: Unusual error of the toast being shown/not shown even if readFromFireStore() is false/true.
    // Maybe can be some reference issue.
    private fun saveInFirestore(device: String) {
        //Firestore document reference
        val docsRef = firestore.collection("Devices Registered").document(device)

        val addDevice: MutableMap<String, String> = HashMap()
        addDevice["UID"] = auth.uid!!
        addDevice["Device Name"] = device


        if (!verifyFirebase()) {
            Timber.d("It already exists!!!!")
        } else {

            docsRef.set(addDevice)
                .addOnSuccessListener {
                    Timber.d("New device saved with name : $device")
                    Toast.makeText(requireContext(), "New Device Saved", Toast.LENGTH_SHORT).show()
                }
                .addOnFailureListener {
                    Toast.makeText(requireContext(), "Not able to save Device", Toast.LENGTH_SHORT)
                        .show()
                    Timber.d("Unable to add device")
                }
        }
    }

    private fun verifyFirebase(): Boolean {

        var valid = true
        val device = binding.etAddDevice.text.toString()
        val docRef = firestore.collection("Devices Registered").document(device)

        docRef.get()
            .addOnSuccessListener { document ->
                if (document != null) {
                    Timber.d("Document data is : ${document.data}")
                    val response = document.getString("Device Name")
                    Timber.d(response)
                    if (response == device) {
                        Timber.d("Device already exists by name : $response")
                        valid = false
                    }
                } else {
                    Timber.d("No such doc exists")
                }
            }
            .addOnFailureListener {
                Timber.d("Failed with exception : $it")
            }
        Timber.d("valid : $valid")
        return valid
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}