Google Sign-In in Android Kotlin App Returns RESULT_CANCELED After Account Selection

I'm working on a Kotlin Android Studio project where I implement Google Sign-In for authentication.
I have set up Firebase with the correct SHA1 and SHA-256, updated the google-services.json file in my project, and created an OAuth client in Google Cloud (using the provided email as default_web_client_id).
Although the Google sign-in popup appears, when I click on an account, the activity immediately shuts down and returns RESULT_CANCELED.

I have included my code below for reference, and any help would be appreciated as I'm relatively new to Android development:

KOTLINCopied
auth = FirebaseAuth.getInstance()
        val db = Firebase.firestore

        val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id))
            .requestEmail()
            .build()

 googleSignInClient = GoogleSignIn.getClient(this, gso)

        findViewById<Button>(R.id.btnSignInWithGoogle).setOnClickListener {
            signInWithGoogle()
        }
/// Functions
 private fun signInWithGoogle()
    {
        val signInIntent = googleSignInClient.signInIntent
        launcher.launch(signInIntent)
    }

    private val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult())
    {result ->
        if(result.resultCode == Activity.RESULT_OK){
           
            val task = GoogleSignIn.getSignedInAccountFromIntent(result.data)
            handleResults(task)
        }
        else if (result.resultCode == Activity.RESULT_CANCELED){
       
        }
    }

    private fun handleResults(task: Task<GoogleSignInAccount>) {
        if (task.isSuccessful) {
            val account: GoogleSignInAccount? = task.result
            updateUI(account)
        } else {
            updateUI(null)

        }
    }

    private fun updateUI(account: GoogleSignInAccount?) {
        if (account != null) {
            val credential = GoogleAuthProvider.getCredential(account.idToken, null)
            auth.signInWithCredential(credential).addOnCompleteListener {
                if (it.isSuccessful) {
                    Log.d(TAG, "signInWithGoogle: Firebase authentication successful")
                    val intent = Intent(this, Register::class.java)
                    startActivity(intent)
                    finish()
                } else {
                    Log.w(TAG, "signInWithGoogle: Firebase authentication failed", it.exception)
                    Toast.makeText(
                        baseContext,
                        "Authentication failed: ${it.exception?.message}",
                        Toast.LENGTH_SHORT
                    ).show()
                }
            }
        } else {
            Log.w(TAG, "Google account is null")
            Toast.makeText(this, "Google account is null", Toast.LENGTH_SHORT).show()
        }
    }