doPost Twice in Google Apps Script? - google-apps-script

Question

I have a web-deployed form written in Google Apps Script with doGet and doPost. In the doPost, the code checks if the user has filled in the form correctly (e.g. not leaving certain things blank). If not, it highlights the things that need to be fixed and adds a warning label. If everything is all right, it writes the form data to a spreadsheet.
The problem is that it doesn't seem like doPost can be called again if the user fixes the problems.
Any thoughts? Thanks!
EDIT: I am using UiService
EDIT: Here is a very simplified version of the app:
function doGet(e) {
var app = UiApp.createApplication();
var mainForm = app.createFormPanel().setId('mainForm');
var formContent = app.createVerticalPanel().setId('formContent');
var userName = app.createTextBox().setId('userName').setName('userName');
var passport = app.createFileUpload().setName('passport');
var submitButton = app.createSubmitButton('submit here');
var submitButtonWarning = app.createLabel('Something is wrong.').setId('submitButtonWarning')
.setVisible(false);
formContent
.add(userName)
.add(passport)
.add(submitButton)
.add(submitButtonWarning);
mainForm.add(formContent);
app.add(mainForm);
return app;
}
function doPost(e) {
var app = UiApp.getActiveApplication();
var userName = e.parameter.userName;
var passport = e.parameter.passport;
if (userName == 'no') {
app.getElementById('submitButtonWarning').setVisible(true);
app.add(app.getElementById('formContent'));
return app;
} else {
app.getElementById('submitButtonWarning').setVisible(false);
app.add(app.getElementById('formContent'));
return app;
}
return app;
}

Answer 1

I think you are just adding the wrong UI element to the app in your doPost.
Instead of
app.add(app.getElementById('formContent'));
use
app.add(app.getElementById('mainForm'));