Google Apps Script Stop Function from Running - google-apps-script

Question

UPDATE: Added doGet function.
I have a web app that is used for a registration process. I created an email confirmation that sends the user an email when they click the Register button.
My question is, how do I stop the function from running if the email field is left blank? At this point when I run the whole script, an error message is shown when I click register if I don't enter an email address. I don't want the function to throw an error and I want to be able to still register.
function doGet(e) {
var app = UiApp.createApplication().setTitle('Education Registration');
app.setStyleAttribute("background", "#DBE8C4");
var panel1 = app.createAbsolutePanel().setId('panel1');
panel1.setHeight(900);
panel1.setWidth(1500);
//Register button
var dateSelection = app.createButton('Register').setSize(140, 40);
var loadingWait = app.createLabel('After clicking Register, please allow 5 - 30 seconds for the webpage to process the request.');
var clickHandler = app.createServerHandler("respondToDateSelection");
dateSelection.addClickHandler(clickHandler);
clickHandler.addCallbackElement(panel1);
//Email Handler
var emailHandler = app.createServerHandler("emailConfirmation");
dateSelection.addClickHandler(emailHandler);
emailHandler.addCallbackElement(panel1);
return app;
}
function emailConfirmation(e) {
var app = UiApp.getActiveApplication();
app.getElementById('fNameText1');
app.getElementById('lNameText1');
app.getElementById('eAddressText1');
app.getElementById('dataItemsLB');
app.getElementById('aemailAddressText');
var fNameText1 = e.parameter.fNameText1;
var lNameText1 = e.parameter.lNameText1;
var eAddressText1 = e.parameter.eAddressText1;
var dataItemsLB = e.parameter.dataItemsLB;
var aemailAddressText = e.parameter.aemailAddressText;
var subject = "Class Registration Confirmation - " + fNameText1 + " " + lNameText1;
var emailBody = "This is an Email Confirmation.";
MailApp.sendEmail(eAddressText1, subject,
emailBody, {cc: aemailAddressText});
return app;
}

Answer 1

There are many ways to do that, the most "elegant" way would probably be using a client handler validator in your doGet function but you didn't show it ...(there is a specific validator for emails)
Another way is to have a warning label in your UI that is initially invisible and that gets visible when e.parameter.eAddressText1 is not a valid email or is empty, the same condition would apply to the sendEmail command and skip it if not valid then return to the UI.
feel free to post your doGet function to allow for more accurate answer.
EDIT :
thanks for posting your code, although it was incomplete and needed some work to make it an interesting example, I end up with this code to illustrate the use of clientHandlers, validators and try/catch for email adress...
Here is the code, I changed it to work on a spreadsheet to avoid deploying and versioning, it is just a test, nothing more...
function doGet(e) {
var app = UiApp.createApplication().setTitle('Education Registration');
app.setStyleAttribute("background", "#DBE8C4");
var panel0 = app.createFlowPanel().setId('panel0');
var panel1 = app.createVerticalPanel().setId('panel1');
var fNameText1=app.createTextBox().setName('fNameText1').setId('fNameText1');;
var lNameText1=app.createTextBox().setName('lNameText1').setId('lNameText1');;
var eAddressText1=app.createTextBox().setName('eAddressText1').setId('eAddressText1').setText('mail')
var dataItemsLB=app.createTextBox().setName('dataItemsLB').setId('dataItemsLB');;
var aemailAddressText=app.createTextBox().setName('aemailAddressText').setId('aemailAddressText').setText('mail');
panel1.add(fNameText1).add(lNameText1).add(eAddressText1).add(dataItemsLB).add(aemailAddressText)
panel0.add(panel1)
//Register button
var dateSelection = app.createButton('Register').setSize(140, 40).setId('dateSelection');
var loadingWait = app.createLabel('After clicking Register, please allow 5 - 30 seconds for the webpage to process the request.').setVisible(false).setId('loadingWait');
var clickHandler = app.createServerHandler("respondToDateSelection").validateEmail(eAddressText1);
dateSelection.addClickHandler(clickHandler);
clickHandler.addCallbackElement(panel0);
//Email Handler
var emailHandler = app.createServerHandler("emailConfirmation").validateEmail(eAddressText1);
dateSelection.addClickHandler(emailHandler);
emailHandler.addCallbackElement(panel1);
//client handlers
var warning = app.createLabel('Please enter your email where necessary').setId('warning').setVisible(false).setStyleAttribute('background','yellow')
var clientHandlerwait = app.createClientHandler().forTargets(loadingWait).setVisible(true).validateEmail(eAddressText1)
var clientHandler1 = app.createClientHandler().validateNotEmail(eAddressText1)
.forTargets(warning).setVisible(true).forEventSource().setStyleAttribute('color','red')
var clientHandler2 = app.createClientHandler().validateNotEmail(aemailAddressText)
.forTargets(warning).setVisible(true).forEventSource().setStyleAttribute('color','red')
dateSelection.addClickHandler(clientHandlerwait).addClickHandler(clientHandler1).addClickHandler(clientHandler2)
app.add(panel1.add(dateSelection).add(loadingWait).add(warning))
SpreadsheetApp.getActive().show(app)
// return app;
}
function respondToDateSelection(){
return
}
function emailConfirmation(e) {
var app = UiApp.getActiveApplication();
app.getElementById('warning').setVisible(false);
app.getElementById('dateSelection').setStyleAttribute('color','black')
var fNameText1 = e.parameter.fNameText1;
var lNameText1 = e.parameter.lNameText1;
var eAddressText1 = e.parameter.eAddressText1;
var dataItemsLB = e.parameter.dataItemsLB;
var aemailAddressText = e.parameter.aemailAddressText;
var subject = "Class Registration Confirmation - " + fNameText1 + " " + lNameText1;
var emailBody = "This is an Email Confirmation.";
try{
// MailApp.sendEmail(eAddressText1, subject,emailBody, {cc: aemailAddressText});
Utilities.sleep(500)// simulate a duration to read the message
app.getElementById('loadingWait').setText('mail sent').setVisible(true)
}catch(err){
app.getElementById('loadingWait').setText('error sending mail').setVisible(true)
}
return app;
}