onActivityResult not called using Google Drive API

I am trying to use the Google Drive API to upload a file to Drive. However, in my code below after I do

startActivityForResult(signInIntent, 400);

It never reaches onActivityResult, it just goes directly to uploadPdfFile() with a null driveServiceHelper.

I have looked into several things I found around here, but this seems to be a different issue? I am a beginner with Android so maybe I am overlooking something? Thanks.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    requestSignIn();
    uploadPdfFile();
} 

private void uploadPdfFile() {
    ProgressDialog progressDialog = new ProgressDialog(MainActivity.this);
    progressDialog.setTitle("Uploading to Google Drive");
    progressDialog.setMessage("Please wait...");
    progressDialog.show();

    String filePath = "/storage/emulated/0/mypdf.pdf";
    driveServiceHelper.createFilePDF(filePath)
            .addOnSuccessListener(new OnSuccessListener<String>() {
                @Override
                public void onSuccess(String s) {
                    progressDialog.dismiss();
                    Toast.makeText(getApplicationContext(), "Upload successfully", Toast.LENGTH_LONG).show();
                }
            })
            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    progressDialog.dismiss();
                    Toast.makeText(getApplicationContext(), "Check your drive api key", Toast.LENGTH_LONG).show();
                }
            });
}

DriveServiceHelper driveServiceHelper;

private void requestSignIn() {
    GoogleSignInOptions signInOptions = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestEmail()
            .requestScopes(new Scope(DriveScopes.DRIVE_FILE))
            .build();
    GoogleSignInClient client = GoogleSignIn.getClient(this, signInOptions);
    Intent signInIntent = client.getSignInIntent();
    startActivityForResult(signInIntent, 400);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    switch (requestCode) {
        case 400:
            if (resultCode == RESULT_OK) {
                handleSignalInIntent(data);
            }
            break;
    }
}

private void handleSignalInIntent(Intent data) {
    GoogleSignIn.getSignedInAccountFromIntent(data)
            .addOnSuccessListener(new OnSuccessListener<GoogleSignInAccount>() {
                @Override
                public void onSuccess(GoogleSignInAccount googleSignInAccount) {
                    GoogleAccountCredential credential = GoogleAccountCredential
                            .usingOAuth2(MainActivity.this, Collections.singleton(DriveScopes.DRIVE_FILE));
                    credential.setSelectedAccountName(googleSignInAccount.getAccount().name);
                    Drive googleDriveService = new Drive.Builder(
                            AndroidHttp.newCompatibleTransport(),
                            new GsonFactory(),
                            credential)
                            .setApplicationName("My Drive Tutorial")
                            .build();

                    driveServiceHelper = new DriveServiceHelper(googleDriveService);
                }
            })
            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                }
            });
}

This is pretty much all the code in the project. (apart from the DriveServiceHelper)

Root cause:
Firstly You are calling requestSignIn method. As soon as line startActivityForResult(signInIntent, 400); executed,
control goes to uploadPdfFile method call. At that time your driveServiceHelper is not initialized.

To handle this, before calling uploadPdfFile method, firstly you have to check whether you are already singin or not. If already signin then only call uploadPdfFile method. else call requestSignIn method. and after this line

driveServiceHelper = new DriveServiceHelper(googleDriveService); 
uploadPdfFile()