selenium capture popup window

i want to capture a popupwindow`s textbox in the div level,but when i run the below code,it displays that:
document.getElementById(...) is null

code follows:
String js1 = "document.getElementById(\"sourceModal\").style.display='block';";
((JavascriptExecutor)driver).executeScript(js1);
driver.findElement(By.id("ChooseHouseHouseID")).sendKeys("212748");