fix(ui): relationship add button unsafe permissions property access (#14903)

### What?
Fix crashing due to unsafe permissions property access in
`AddNewRelation` by safely checking that a collection’s permissions
exist before reading its `create` property.

### Why?
Accessing `permissions.collections[slug].create` throws and breaks the
UI when a collection’s permissions entry is missing.
This happens for example when collection‑level access control makes that
collection inaccessible.

### How?
Change the condition to use optional chaining on the permissions,
aligning with usage elsewhere in the codebase:
- Before: `permissions.collections[relatedCollection?.slug].create`
- After: `permissions.collections[relatedCollection?.slug]?.create`

Author: AaronClaes

packages/ui/src/elements/AddNewRelation/index.tsx

@@ -193,7 +193,7 @@ export const AddNewRelation: React.FC<Props> = ({
             render={({ close: closePopup }) => (
               <PopupList.ButtonGroup>
                 {relatedCollections.map((relatedCollection) => {
-                  if (permissions.collections[relatedCollection?.slug].create) {
+                  if (permissions.collections[relatedCollection?.slug]?.create) {
                     return (
                       <PopupList.Button
                         className={`${baseClass}__relation-button--${relatedCollection?.slug}`}