Bug: 'Add a Registrant' dropdown is empty for invite-only events and crashes on text input

## Description When an event creator opens the **"Add a Registrant"** dialog for an **invite-only** event, the user-search dropdown is empty (no users listed). Typing any text into the search field (e.g., "harve") causes a JavaScript crash: ``` Cannot read properties of null (reading 'removeAttribute') ``` This affects both **Admin Portal** and **User Portal**, and reproduces for both admin and regular user logins. ## Steps to Reproduce 1. Create an event with **Invite Only** visibility 2. Open the event (Admin Portal or User Portal) 3. Click **"Add a Registrant"** 4. Observe: the user-search dropdown is **empty** — no users are listed 5. Type any characters in the search field (e.g., `harve`) 6. Observe crash: `Cannot read properties of null (reading 'removeAttribute')` ## Expected Behavior - The dropdown should list searchable users that can be added as registrants - Typing in the search field should filter users without crashing - The event creator (and admins) should be able to register attendees for invite-only events ## Actual Behavior - Dropdown shows no users at all - Typing triggers a null-dereference crash: `Cannot read properties of null (reading 'removeAttribute')` ## Impact - **Severity:** High (complete loss of registrant-management functionality for invite-only events) - **Affected flows:** Admin Portal event management, User Portal event registration management - **Affected roles:** Both admin and regular user logins ## Notes - This bug is **pre-existing** and was **not introduced** by PR `#7440` - Discovered during manual testing of invite-only event flows while working on PR `#7440` - Reference: https://github.com/PalisadoesFoundation/talawa-admin/pull/7440 ## Files Likely Involved - Component(s) rendering the "Add a Registrant" modal/dropdown - User-search query / GraphQL query used to populate the registrant dropdown - The null-dereference in `removeAttribute` suggests a DOM node is being accessed after unmount or before mount — likely in the autocomplete/combobox input handler