/**
* Checks if title is valid.
*
* @return error message or NULL.
*/
protected String validateTitle() {
String message = null;
final String title = tfTitle.getText();
if (title == null || title.trim().length() == 0) {
message = Strings.message("guide.dialog.validation.empty.title");
} else if (CommonUtils.areDifferent(originalTitle, title) && presentTitles.contains(title)) {
message = Strings.message("guide.dialog.validation.already.present");
}
return message;
}
