مشکل با برگشت در webview

سلام میخوام با استفاده از دکمه back روی گوشی در webview به عقب برگردم و در نهایت که به صفحه اول برگشت با زدن یه بار دگ دکمه back از webview خارج بشه

کدش رو نوشتم فقط یه مشکل دارم وقتی اتصال به اینترنت ندارم و دکمه بک رو میزنم برنامه کرش میکنه!!!! واقعا نمیدونم دلیلش چیه :(((

 package r.ali.webview;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.util.Log;
import android.view.Window;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;


public class WebviewActivity extends Activity {

    private WebView             webview;
    private static final String TAG = "Main";
    private ProgressDialog      progressBar;


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

        requestWindowFeature(Window.FEATURE_NO_TITLE);

        setContentView(R.layout.main);
        if (isNetworkAvailable(getApplicationContext()))
        {
            loadingUrl();
        }
        else {
            Toast.makeText(getApplicationContext(), "connection lost", 3000).show();
        }
    }


    /*----------------------------------------------------------------------------------------------------------*/

    private void loadingUrl() {
        this.webview = (WebView) findViewById(R.id.webview);

        WebSettings settings = webview.getSettings();
        settings.setJavaScriptEnabled(true);
        webview.setScrollBarStyle(WebView.SCROLLBARS_OUTSIDE_OVERLAY);

        final AlertDialog alertDialog = new AlertDialog.Builder(this).create();

        progressBar = ProgressDialog.show(WebviewActivity.this, "WebView Example", "Loading...");

        webview.setWebViewClient(new WebViewClient() {

            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
                Log.i(TAG, "Processing webview url click...");
                view.loadUrl(url);
                return true;
            }


            @Override
            public void onPageFinished(WebView view, String url) {
                Log.i(TAG, "Finished loading URL: " + url);
                if (progressBar.isShowing()) {
                    progressBar.dismiss();
                }
            }


            @Override
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Log.e(TAG, "Error: " + description);
                Toast.makeText(WebviewActivity.this, "Oh no! " + description, Toast.LENGTH_SHORT).show();
                alertDialog.setTitle("Error");
                alertDialog.setMessage(description);
                alertDialog.setButton("OK", new DialogInterface.OnClickListener() {

                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        return;
                    }
                });
                alertDialog.show();
            }
        });
        webview.loadUrl("http://www.google.com");
    }


    /*----------------------------------------------------------------------------------------------------------*/
    @Override
    public void onBackPressed()
    {
        if (webview.isFocused() && webview.canGoBack())
        {
            webview.goBack();
        }
        else
        {
            super.onBackPressed();
            finish();
        }

    }


    /*----------------------------------------------------------------------------------------------------------*/
    public boolean isNetworkAvailable(Context context) {
        if (context == null) {
            return false;
        }
        ConnectivityManager connectivityManager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        // if no network is available networkInfo will be null, otherwise check if we are connected
        try {
            NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
            if (activeNetworkInfo != null && activeNetworkInfo.isConnected()) {
                return true;
            }
        }
        catch (Exception e) {
            Log.i(TAG, "isNetworkAvailable()");
        }
        return false;
    }
    /*----------------------------------------------------------------------------------------------------------*/

}

اینم کد xml :

 <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >



    <WebView
        android:id="@+id/webview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>