You don't specify which line is actually throwing the error, but it probably either the collection-filter or the foreach loop.

In either of those two lines, conversation_interlocutors doesn't exist. That means the user may or may not have a conversation attached? Maybe a message was deleted 'the wrong way' so your system crashes?

Maybe the easy solution is to just check if the conversation_interlocutors exist before returning and pushing them:

$filtered = $collections->filter(function ($collection) {
    if (isset($collection->user->conversation_interlocutors)) {
        return $collection->user->conversation_interlocutors;
    }
});

foreach ($filtered as $filter) {
    if (isset($filter->user->conversation_interlocutors)) {
        $conversations->push($filter->user->conversation_interlocutors);
    }
}

That way, you're making sure you are only working with existing data, even if the database has been corrupted somehow.