Alert box closes my application

Question

When I run this application it crashes if there is no Internet connectivity.
Why?

I have a separate Class in which I have a method to provide Internet details.
Internet permission is set in the Manifest

public class MainActivity extends Activity {

private WebView webView;
private AlertDialog.Builder alertDiaBuilder;
private ProgressDialog progressBar;
private int netState = 0;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    webView = (WebView) findViewById(R.id.webView1);

    progressBar = new ProgressDialog(MainActivity.this);

    progressBar.setCancelable(false);

    progressBar.setTitle("Checking network connectivity");

    progressBar.setMessage("Wait for a movement!");

    progressBar.setProgressStyle(ProgressDialog.STYLE_SPINNER);

    progressBar.show();

    new Thread(new Runnable() {

        public void run() {

            netState = NetworkUtils.getConnectivityStatus(MainActivity.this);

            if (netState == 1 || netState == 2 || netState == 3
                    || netState == 4) {

                try {

                    Thread.sleep(5000);

                    progressBar.dismiss();

                    webView.loadUrl("http://www.yahoo.co.in");

                } catch (InterruptedException e) {

                    e.printStackTrace();
                }
            } else {

                alertDiaBuilder = new AlertDialog.Builder(MainActivity.this);
                alertDiaBuilder.setMessage("Not Connected to Internet");

                alertDiaBuilder.setCancelable(false);

                alertDiaBuilder.setNegativeButton("Close application",

                        new OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog,
                                    int which) {

                                finish();
                                //dialog.cancel();
                            }
                        });

                alertDiaBuilder.setPositiveButton("Go offline",
                        new OnClickListener() {

                            @Override
                            public void onClick(DialogInterface dialog,
                                    int which) {

                                Toast.makeText(getApplicationContext(),
                                        "Ready to go offline",
                                        Toast.LENGTH_LONG).show();
                            }
                        });

                AlertDialog alertDialog = alertDiaBuilder.create();

                alertDialog.show();
            }
        }

    }).start();
}
}