In the above sample code, we are transferring a value of Java/Groovy variable of java.lang.String into a String literal in JavaScript. You need to be careful for translating special characters such as NEWLINE, quotes, etc. There are a lot of more characters you need to look after. For example, the sample code can not deal with a backslash character \ appropriately. If the input text contains one or more \, the code will break. You have to modify the code to escape more special characters. This problem sucks.
An alternative is available. As I mentioned in the post
You would be able to edit the content text of the <body contenteditable="true"> element in your target web page using WebUI.click(), WebUI.clear() and WebUI.sendKeys(). This way you do not have to look after escaping special characters. The WebDriver protocol implentation (e.g. ChromeDriver, FirefoxDriver) will look after it for you.