Laravel 5.6: ErrorException: compact(): Undefined variable: operator

I am upgrading my laravel from 5.5 to 5.6. and upon running tests, I am getting this error:

ErrorException: compact(): Undefined variable: operator

Now there is a solution present here Laravel - compact(): Undefined variable: operator in Builder.php but I am following that very solution and still facing that error. Earlier I was upgrading from 5.4 to 5.6 then upon seeing the solution mentioned in the above question, I upgraded from 5.4 to 5.5 and now trying to upgrade from 5.5 to 5.6.

Here is a little piece of code that is causing this error:

$query = TransportOrder::with('orderNumber');

$query->whereHas('orderNumber', function ($q) use ($orderNumber) {
      $q->where('order_number', '=', $orderNumber);
});

return $query->first();

That whereHas is causing this problem. So is there an alternate to whereHas and with or there is any other solution that I am missing?

My php version is this:

PHP 7.3.23-1+ubuntu18.04.1+deb.sury.org+1 (cli) (built: Oct  6 2020 11:36:27) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.23, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.23-1+ubuntu18.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies
    with blackfire v1.24.2~linux-x64-non_zts73, https://blackfire.io, by Blackfire

EDIT:

compact() is used in vendor class Builder:

public function addWhereExistsQuery(self $query, $boolean = 'and', $not = false)
    {
        $type = $not ? 'NotExists' : 'Exists';

        $this->wheres[] = compact('type', 'operator', 'query', 'boolean');

        $this->addBinding($query->getBindings(), 'where');

        return $this;
    }

As it turns out upgrading laravel to 6.5.40 did the trick, as the mentioned issue was not resolved in the earlier versions.