Event scripting issues

Question

I am trying to get the following script working as a startup script.
the goal is that when the user chooses to save the document, they are presented with a dialog box asking for a quick summary of the edits that were made. Then this info is written to XMP fields.
The core functionality actually works, the dialog prompts and the data gets saved in the XMP.
The problem is that when the save action is initiated an error dialog pops up announcing "The requested action could not be completed because the object no longer exists." and then the script proceeds and functions properly, then it prompts for input and saves the data again.
I get an error, then the script runs twice.
I am confused.
#targetengine session
var myEventListener = app.addEventListener("beforeSave", myPromptForInput);
function myPromptForInput(myEvent){
var myTimeStamp = myEvent.timeStamp
var myDialog = app.dialogs.add({name:"Enter a simple description of edits made",canCancel:true});
with(myDialog){
//Add a dialog column.
with(dialogColumns.add()){
//Create a text edit field.
var myTextEditField = textEditboxes.add({editContents:"Description of document changes", minWidth:300});
//Display the dialog box.
var myResult = myDialog.show();
if(myResult == true){
//Get the values from the dialog box controls.
var myNotes = myTextEditField.editContents;
//Remove the dialog box from memory.
myDialog.destroy();
myAddXMPData(myNotes + " - " + myTimeStamp);
//alert(myEvent);
function myAddXMPData(myNotes){ 
//var myDocument = app.documents.item(0);
var myDocument = app.activeDocument;
with(myDocument.metadataPreferences){
author = "James Haney";
description = myNotes;
var myNewContainer = createContainerItem("http://ns.adobe.com/xap/1.0/", "LCCRevisionNotes");
setProperty("http://ns.adobe.com/xap/1.0/", "LCCRevisionNotes/*[1]", myNotes);

Answer 1

@Jarek – I tested your code as well.
Even without starting Illustrator I can see one warning message. The script is running fine as is.
If I add something to InDesign's XMP data and do a document save your editing window is showing up and I can edit a text field.
After hitting ok, a warning message is coming up.
1. Unsaved named document.
2. After working the keyboard shortcut for save cmd + s your text edit field is coming up:
3. Adding some text:
4. Warning message is popping up after hitting OK:
This warning message is quite different from James' error message:
Warning
An attached script is causing the following error:
myAddXMPData is not a function
Would you like to disable this event handler?
[ ] Do not show again
[No] [[Yes]]
Tested with InDesign CC 2014.2 v10.2.0.69 on Mac OSX 10.7.5.
Script is saved in folder:
Applications/Adobe InDesign 2014/Scripts/startup scripts/Get-myXMP-atSave-Dummy.jsx
Uwe