Can't access OAuth Access token method while doing Github Login with Firebase

According to Firebase documentation to get OAuth access token on successful login, we can get it from the getCredential() method like: authResult.getCredential().getAccessToken()

Code: According to firebase:

firebaseAuth
.startActivityForSignInWithProvider(/* activity= */ this, provider.build())
.addOnSuccessListener(
    new OnSuccessListener<AuthResult>() {
      @Override
      public void onSuccess(AuthResult authResult) {
        // User is signed in.
        // IdP data available in
        // authResult.getAdditionalUserInfo().getProfile().
        // The OAuth access token can also be retrieved:
        // authResult.getCredential().getAccessToken().
      }
    })
.addOnFailureListener(
    new OnFailureListener() {
      @Override
      public void onFailure(@NonNull Exception e) {
        // Handle failure.
      }
    });

But, when I try to access the token like that, it just simply says that the method doesn’t exist.

 pendingResultTask
    ?.addOnSuccessListener { authResult ->
        val username = authResult.additionalUserInfo?.username
        val accessToken = authResult.getCredential().getAccessToken()

        Toast.makeText(
            activity,
            "username: $username, accessToken: $accessToken",
            Toast.LENGTH_LONG
        ).show()
        navigateToHomeScreen()
    }
    ?.addOnFailureListener { exception ->
        Toast.makeText(
            activity,
            "Something Went Wrong: ${exception.message}",
            Toast.LENGTH_LONG
        ).show()
    }

here in this code, I can’t use getAccessToken() method, error: Unresolved reference

Why I am not able to access the getAccessToken() method?