DateBox : Null value not identified

I have to do mandatory check for a date fieldWhen the user doesn't any
date in the datebox input

Code:
final DateBox dateBox = new DateBox();
dateBox.setValue(new Date());
dateBox.setFormat(new
DateBox.DefaultFormat(DateTimeFormat.getFormat("ddMMMyy")));

//capture the change event of date picker
dateBox.addValueChangeHandler(new ValueChangeHandler<Date>() {
@Override
public void onValueChange(final ValueChangeEvent<Date> event) {
if (event.getValue() == null) {
Window.alert("Date cannot be null");
validationFailed = true;
} }
});

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