Question

Why app.getElementById('myTextBox').setValue('default') work in showDialog() but didnt work in respondToSubmit(e)?

function showDialog() {
  var app = UiApp.createApplication();
  var panel = app.createVerticalPanel();

  var textBox = app.createTextBox();
  textBox.setName('myTextBox').setId('myTextBox');
  app.getElementById('myTextBox').setValue('default');

  var button = app.createButton(Modify');
  panel.add(textBox);
  panel.add(button);

  var clickHandler = app.createServerClickHandler("respondToSubmit");
  button.addClickHandler(clickHandler);
  clickHandler.addCallbackElement(panel);

  app.add(panel);
  var doc = SpreadsheetApp.getActive();
  doc.show(app);
}

function respondToSubmit(e) {
  var app = UiApp.getActiveApplication();
  app.getElementById('myTextBox').setText('Modifed');
}

Answer 1

Add

return app; 

at the end of your respondToSubmit function