Did you consider using jsExecutor?
JavascriptExecutor js = (JavascriptExecutor) driver;
StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append("var combo=Ext.getCmp('combobox name');");
stringBuilder.append("combo.setValue('4');");
stringBuilder.append("combo.fireEvent('select');");
js.executeScript(stringBuilder.toString());
If any other exceptions are thrown, try debugging step by step in your IDE.
You may also find helpful information here under the Executing JavaScript from Your Test section:
JavaScript can be very useful for testing applications not directly supported by selenium. The getEval method of the selenium API can execute JavaScript from selenium RC.
public static String[] getAllCheckboxIds () {
String script = "var inputId = new Array();";// Create array in java script.
script += "var cnt = 0;"; // Counter for check box ids.
script += "var inputFields = new Array();"; // Create array in java script.
script += "inputFields = window.document.getElementsByTagName('input');"; // Collect input elements.
script += "for(var i=0; i<inputFields.length; i++) {"; // Loop through the collected elements.
script += "if(inputFields[i].id !=null " +
"&& inputFields[i].id !='undefined' " +
"&& inputFields[i].getAttribute('type') == 'checkbox') {"; // If input field is of type check box and input id is not null.
script += "inputId[cnt]=inputFields[i].id ;" + // Save check box id to inputId array.
"cnt++;" + // increment the counter.
"}" + // end of if.
"}"; // end of for.
script += "inputId.toString();" ;// Convert array in to string.
String[] checkboxIds = selenium.getEval(script).split(","); // Split the string.
return checkboxIds;
}
Here's another example:
selenium.getEval("window.document.images.length;");
I hope this information is helpful to you.