无法将JavaScript代码嵌入到Android网页视图中？

Question

我想将javascript代码嵌入到原始的facebook页面中。但是我收到了错误

  

“Uncaught TypeError：无法设置null”属性'value'

请帮助我。这是我的代码。

public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    WebView webView=(WebView)findViewById(R.id.wv);
    webView.getSettings().setJavaScriptEnabled(true);
    webView.setWebViewClient(new WebViewClient(){
        @Override
        public void onPageFinished(WebView view, String url) {
            super.onPageFinished(view, url);
            String email="qwerty@xyz.com";
            String password="1234";
            String javaScript="javascript:(function(){ document.getElementById('email').value='"+email+
                    "';document.getElementById('pass').value='"+password+
                    "';document.getElementById('u_0_a').click();})()";
            view.loadUrl(javaScript);
        }
    });
    webView.loadUrl("http://www.facebook.com");

}

}