NetworkOnMainThreadException - connecting to Dropbox account

I followed the tutorial on file uploading from the Dropbox website here: https://www.dropbox.com/developers/documentation/java#tutorial 

But I am having problems when running my code. 

Here is my error: 

W/System.err: android.os.NetworkOnMainThreadException
W/System.err: at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1273)
W/System.err: at java.net.InetAddress.lookupHostByName(InetAddress.java:431)
W/System.err: at java.net.InetAddress.getAllByNameImpl(InetAddress.java:252)
W/System.err: at java.net.InetAddress.getAllByName(InetAddress.java:215)
W/System.err: at com.android.okhttp.internal.Network$1.resolveInetAddresses(Network.java:29)
W/System.err: at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:188)
W/System.err: at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:157)
W/System.err: at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:100)
W/System.err: at com.android.okhttp.internal.http.HttpEngine.createNextConnection(HttpEngine.java:357)
W/System.err: at com.android.okhttp.internal.http.HttpEngine.nextConnection(HttpEngine.java:340)
W/System.err: at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:330)
W/System.err: at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:248)
W/System.err: at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:433)
W/System.err: at com.android.okhttp.internal.huc.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:114)
W/System.err: at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getOutputStream(HttpURLConnectionImpl.java:245)
W/System.err: at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getOutputStream(DelegatingHttpsURLConnection.java:218)
W/System.err: at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java)
W/System.err: at com.dropbox.core.http.StandardHttpRequestor.getOutputStream(StandardHttpRequestor.java:123)
W/System.err: at com.dropbox.core.http.StandardHttpRequestor.access$000(StandardHttpRequestor.java:28)
W/System.err: at com.dropbox.core.http.StandardHttpRequestor$Uploader.<init>(StandardHttpRequestor.java:133)
W/System.err: at com.dropbox.core.http.StandardHttpRequestor.startPost(StandardHttpRequestor.java:72)
W/System.err: at com.dropbox.core.http.StandardHttpRequestor.startPost(StandardHttpRequestor.java:28)
W/System.err: at com.dropbox.core.v2.DbxRawClientV2.uploadStyle(DbxRawClientV2.java:221)
W/System.err: at com.dropbox.core.v2.files.DbxUserFilesRequests.upload(DbxUserFilesRequests.java:1261)
W/System.err: at com.dropbox.core.v2.files.UploadBuilder.start(UploadBuilder.java:114)
W/System.err: at com.dropbox.core.v2.files.UploadBuilder.start(UploadBuilder.java:18)
W/System.err: at com.dropbox.core.v2.DbxUploadStyleBuilder.uploadAndFinish(DbxUploadStyleBuilder.java:92)
W/System.err: at com.example.kestites.exportdatabase.MainActivity$1.onClick(MainActivity.java:89)
W/System.err: at android.view.View.performClick(View.java:5198)
W/System.err: at android.view.View$PerformClick.run(View.java:21147)
W/System.err: at android.os.Handler.handleCallback(Handler.java:739)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:95)
W/System.err: at android.os.Looper.loop(Looper.java:148)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5417)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Here is my code:

public class MainActivity extends AppCompatActivity {

    private Button upload;
    private static final String ACCESS_TOKEN = "<APP_KEY>";
    DbxRequestConfig config = new DbxRequestConfig("dropbox/java-tutorial", "en_US");
    DbxClientV2 client = new DbxClientV2(config, ACCESS_TOKEN);
    private String outputFile = null;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ActivityCompat.requestPermissions(MainActivity.this, new String[] {android.Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        upload = (Button)findViewById(R.id.button);
        upload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {



                File patternDirectory = new File(Environment.getExternalStorageDirectory().getAbsolutePath().toString()+"/test.txt");
                patternDirectory.mkdirs();
                String filename = "test.txt";
                outputFile = Environment.getExternalStorageDirectory() + "/test.txt";
                String string = "Hello world!";
                FileOutputStream outputStream;

                try {
                    outputStream = new FileOutputStream(new File(patternDirectory.getAbsolutePath().toString()+"/test.txt"),true);
                    outputStream.write(string.getBytes());
                    outputStream.close();
                } catch (Exception e) {
                    System.out.println("Creating file not working");
                    e.printStackTrace();
                }



                try {
                    InputStream in = new FileInputStream(patternDirectory.getAbsolutePath().toString()+"/test.txt");
                    FileMetadata metadata = client.files().uploadBuilder("/test.txt").uploadAndFinish(in);

                } catch (Exception e) {
                    System.out.println("Upload file not working");
                    e.printStackTrace();
                }
            }
        });
    }

}