Add null safety checks to prevent PHP warnings

[GaryJones]

Problem

Four places in the codebase lack null/undefined checks, producing PHP warnings or deprecation notices on PHP 8.1+.

1. Undefined title_required property on activity objects

In bp-groups/bp-groups-activity.php, when updating a group activity, the code accesses $activity->title_required without a fallback. If the property doesn't exist on the activity object, PHP emits an undefined property warning.

Fix: Add ?? false null-coalescing fallback.

2. Null passed to wp_kses_post() from notification icon

In bp-notifications/bp-notifications-functions.php, bb_notification_small_icon() can return null, which is then passed directly to wp_kses_post(). On PHP 8.1+, this triggers a deprecation warning since wp_kses_post() expects a string.

Fix: Add ?? '' null-coalescing fallback.

3. Missing progress data in profile completion widget

In class-bp-xprofile-profile-completion-widget.php, the widget accesses $user_progress['completion_percentage'] without checking whether $user_progress is populated or the key exists. When no applicable profile fields are configured, this produces undefined key warnings.

Fix: Add an early return when $user_progress is empty or missing the completion_percentage key.

4. Uninitialised $result variable in topics manager

In class-bb-topics-manager.php, the $result variable is only assigned inside a conditional block (if ( 'migrate' === $migrate_type && $new_topic_id )). When the condition is false, $result is undefined for any subsequent code that references it.

Fix: Initialise $result = array() before the conditional.

Test plan

• Update a group activity where title_required is not set on the activity object — no PHP warning
• View notifications where bb_notification_small_icon() returns null — no deprecation notice
• Display the profile completion widget with no applicable xprofile fields configured — no undefined key warnings
• Use the topics manager with a non-migrate action — no undefined variable notice