Nintex Forms - getting list items by javascript

Dear Community, 

Couldn't find any posts that helped me, so I create a new one...

I'm trying to get item information from a list based on a lookup field. 

So I implemented a little javascript that's fired by a button.

function updateSupplierTemplate(){
var ctx = new SP.ClientContext.get_current();
var myList = ctx.get_web().get_lists().getByTitle('SupplierTemplates');
ctx.load(myList);
var templateID = NWF.RuntimeFunctions.parseLookup(NWF$("#" + supplierTemplate).val(), false);
var myItem = myList.getItemById(templateID);
ctx.load(myItem);
ctx.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}
function onQuerySucceeded() {
alert('success');
alert(myItem);
NWF$("#" + supplierStreet).val(sStreet);
}
function onQueryFailed(sender, args) {
alert('Request failed. Error: ' + args.get_message() + ' StackTrace: ' + args.get_stackTrace());
}

I could see with an alert box, that my lookupID variable is set fine. 

Also the SP.Context must be available because it returns an [object] in an alert box.

The query goes in success function what I checked with an alert action. But as soon as I want to access myItem it returns "object not defined" in the developer function of the browser. 

Is this not a supported way to work with SP items inside a Nintex form?

Thanks for any help in advance.

Rubi