Fix: Add null check for notification object in ReplaceNotificationEvent

[prjanitor]

Summary

This PR adds a null check for the notification object in the ReplaceNotificationEvent case of the _notificationEventListener method.

Problem

The ReplaceNotificationEvent case was calling onNotificationReplaced(oldId, id) without verifying that the new notification exists. If the notification service returns null for a non-existent notification ID, this could lead to unexpected behavior or crashes downstream.

Solution

Added the same null-check pattern used in ShowNotificationEvent:

• Retrieve the notification using service.getNotification(id)
• Return early if the notification is null
• Only call onNotificationReplaced if the notification exists

Testing

This change ensures consistency with the existing null-check pattern in ShowNotificationEvent and prevents potential null pointer issues when notification IDs are invalid or stale.

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