This fixes the following error when focusedIndex=-1 and the Enter key is pressed on a dropdown.

Cannot read properties of undefined (reading 'querySelector') TypeError: Cannot read properties of undefined (reading 'querySelector')
at HTMLUListElement._handleDropdownKeydown

Proposed changes

const focusedElement = this.dropdownEl.children[this.focusedIndex];
const activatableElement = focusedElement.querySelector('a, button');
->
const focusedElement = this.dropdownEl.children[this.focusedIndex];
const activatableElement = focusedElement?.querySelector('a, button');

activatableElement is null checked but focusedElement was not.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My commit messages follows the conventional commit format
• My change requires a change to the documentation, and updated it accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.