My webview module isn't working. Whenever I try to navigate back from any webpage, the app crashes giving following error

 "Unfortunately the app has stopped".

Can anyone please explain what is the reason?

public class MainActivity extends ActionBarActivity {
WebView myWebView;
@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    WebView mainWebView = (WebView) findViewById(R.id.mainWebView);

    WebSettings webSettings = mainWebView.getSettings();
    webSettings.setJavaScriptEnabled(true);

    mainWebView.setWebViewClient(new MyCustomWebViewClient());
    mainWebView.setScrollBarStyle(View.SCROLLBARS_INSIDE_OVERLAY);

    mainWebView.loadUrl("http://nahdaschools.com/v4/index.php/apps");
}

private class MyCustomWebViewClient extends WebViewClient
{
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, String url)
    {
        view.loadUrl(url);
        return true;
    }

}

@Override
public void onBackPressed() {
    if (myWebView.copyBackForwardList().getCurrentIndex() > 0) {
        myWebView.goBack();
    }
    else {
        // Your exit alert code, or alternatively line below to finish
        super.onBackPressed(); // finishes activity
    }
}}

   @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

     if(event.getAction() == KeyEvent.ACTION_DOWN){
         switch(keyCode)
         {
         case KeyEvent.KEYCODE_BACK:
         if(myWebView.canGoBack()){
              myWebView.goBack();
        }
       else{
             finish();
          }
            return true;
        }

         }
      return super.onKeyDown(keyCode, event);
           }

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