Can anyone give me a hand with Android's WebView?

I'm trying to hide the top banner of a webpage in an Android WebView, but it's doing my head in! I've been at it for days and still can't get it to work.

I have used the code below, to try to disable the displaying of a certain element, the page loads (including the element I don't want) but then disappears completely and shows the word "none".

private class MyWebViewClient extends WebViewClient {

    @Override
    public boolean shouldOverrideUrlLoading(WebView webView, String url) {
        webView.loadUrl(url);
        return true;
    }

    @Override
    public void onPageFinished(WebView webView, String url) {
        webView.loadUrl("javascript:document.getElementsByClassName('welcome')[0].style.display='none';");
    }

}

This is the onCreate

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    AdView mAdView;
    mAdView = (AdView) findViewById(R.id.adView);
    AdRequest adRequest = new AdRequest.Builder().build();
    mAdView.loadAd(adRequest);

    this.webView = (WebView) findViewById(R.id.webview);

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

    // To keep browser in width of phone
    webView.getSettings().setUseWideViewPort(true);
    webView.getSettings().setLoadWithOverviewMode(true);

    // Sets default zoom
    webView.getSettings().setTextZoom(250);

    //webSettings.setTextZoom(125);

    MyWebViewClient webViewClient = new MyWebViewClient();
    webView.setWebViewClient(webViewClient);

    webView.loadUrl("http://www.example.com");
}

Any class I choose (not just welcome) hides the entire website and displays the word 'none'. And if I change it to try to change height to 0px, it displays '0px' to the screen.

It's not a problem with the specific website I'm using, it does the same for my personal simple HTML / CSS website. Which makes me think either my JavaScript is wrong or there's something in WebView stopping me.