p-dropdown unable to auto-select first item if group used

[contra1337]

Describe the bug

When using p-dropdown with a grouped data set, inside of a reactive form, where autoDisplayFirst is true / omit, and optionGroupChildren is not the default value of 'items', the following error is logged in the browser console and no default item is selected:

core.mjs:8506 ERROR TypeError: Cannot read properties of undefined (reading '0') at Dropdown.updateSelectedOption (primeng-dropdown.mjs:355:69) at Dropdown.ngOnInit (primeng-dropdown.mjs:215:14) at callHook (core.mjs:2488:22) at callHooks (core.mjs:2457:17) at executeInitAndCheckHooks (core.mjs:2408:9) at refreshView (core.mjs:10434:21) at refreshEmbeddedViews (core.mjs:11434:17) at refreshView (core.mjs:10443:9) at refreshEmbeddedViews (core.mjs:11434:17) at refreshView (core.mjs:10443:9) core.mjs:8506 ERROR TypeError: Cannot read properties of undefined (reading '0') at Dropdown.updateSelectedOption (primeng-dropdown.mjs:355:69) at Dropdown.writeValue (primeng-dropdown.mjs:340:14) at setUpControl (forms.mjs:2941:23) at FormGroupDirective.addControl (forms.mjs:4745:9) at FormControlName._setUpControl (forms.mjs:5295:43) at FormControlName.ngOnChanges (forms.mjs:5240:18) at FormControlName.rememberChangeHistoryAndInvokeOnChangesHook (core.mjs:1574:14) at callHook (core.mjs:2498:18) at callHooks (core.mjs:2457:17) at executeInitAndCheckHooks (core.mjs:2408:9)

it looks like the reason is the wrong property is accessed in this one spot in

primeng/src/app/components/dropdown/dropdown.ts

Line 776 in 085a4e6

this.selectedOption = this.optionsToDisplay[0].items[0];

all areas appear to use getOptionGroupChildren() to get the defined field except this one area is still hard coded to 'items'

I'm happy to provide a PR sometime later today to address if the issue can be confirmed. Stack blitz attached, changing the data from children -> items works but should not be required

Environment

MacOS 12.0.1
Chrome 109

Reproducer

https://stackblitz.com/edit/github-ixgaz2?file=src%2Fapp%2Fapp.component.html

Angular version

15.1.0

PrimeNG version

15.1.0

Build / Runtime

Angular CLI App

Language

TypeScript

Node version (for AoT issues node --version)

18.10.0

Browser(s)

Chrome 109

Steps to reproduce the behavior

1. create a simple form control or form group
2. create an options array with groups where the children property is not called 'items' but something like 'children'
3. create a p-dropdown and bind the options
4. set [group]="true"
5. set optionGroupChildren to the children field (again not called items)
6. set the optionGroupLabel and optionLabel if needed
7. bind the formControlName to the same one created in step 1
8. open the dropdown in the browser and see the error

Expected behavior

No error should be logged when the component is using a default value for autoDisplayFirst and group=true and data does not use field items as the child property

The first selectable option within the first group should be selected probably, as works when it is called items