title: ElementNotFoundException in HtmlUnit, although the element exists
body: <p>I've made a java server that scrapes a website, but my problem is that after a few requests (about 10 or so) I always get this error <code>ElementNotFoundException</code>, although the element should be there. Basically my program just checks every few minutes this website for info but after a few times it just gives me that exception. 
  This is my code for scraping, I don't know what's wrong with it that after a few times the element is not found..</p>
  
  <pre><code>final WebClient webClient = new WebClient();
  try (final WebClient webClient1 = new WebClient()) {
      final HtmlPage page = webClient.getPage("http://b7rabin.iscool.co.il/מערכתשעות/tabid/217/language/he-IL/Default.aspx");
  
      WebResponse webResponse = page.getWebResponse();
      String content = webResponse.getContentAsString();
       //   System.out.println(content);
  
  
      HtmlSelect select = (HtmlSelect) page.getElementById("dnn_ctr914_TimeTableView_ClassesList");
      HtmlOption option = select.getOptionByValue("" + userClass);
  
      select.setSelectedAttribute(option, true);
  
      //String jscmnd = "javascript:__doPostBack('dnn$ctr914$TimeTableView$btnChangesTable','')";
      String jscmnd = "__doPostBack('dnn$ctr914$TimeTableView$btnChanges','')";
  
      ScriptResult result = page.executeJavaScript(jscmnd);
  
      HtmlPage page1 = (HtmlPage) result.getNewPage();
  
      String content1 = page1.getWebResponse().getContentAsString();
      //System.out.println(content1);
      System.out.println("-----");
      HtmlDivision getChanges = null;
      String changes = "";
  
      getChanges = page1.getHtmlElementById("dnn_ctr914_TimeTableView_PlaceHolder");   
      changes = getChanges.asText();
      changes = changes.replaceAll("\n", "").replaceAll("\r", "");
  
      System.out.println(changes);
  }
  </code></pre>
  
  <p>The exception:</p>
  
  <pre><code>Exception in thread "Thread-0" com.gargoylesoftware.htmlunit.ElementNotFoundException: elementName=[*] attributeName=[id] attributeValue=[dnn_ctr914_TimeTableView_PlaceHolder]
  at com.gargoylesoftware.htmlunit.html.HtmlPage.getHtmlElementById(HtmlPage.java:1552)
  at scrapper$1.run(scrapper.java:108)
  </code></pre>
  
  <p>I am really desperate to solve it, it's the only bottleneck in my project.</p>