Laravel excel export headings() throw exception only in queue

Question

I am trying to export large data using Laravel Excel Queued. It works fine if i remove the headings method from the export class. If i try to include headings method it give me an exception.

FatalThrowableError: Return value of App\Exports\DataTablesExport::headings() must be of the type array, null returned in DataTablesExport.php:72

In My Controller

/**
   * @var $query \Illuminate\Database\Eloquent\Builder
     */

    $columns = array_keys($query->first()->toArray());

    (new DataTablesExport($query, $columns))->queue($file)->chain([
        new NotifyAdminOfCompletedExport(request()->user(), $file)
    ]);

In Export Class

<?php

namespace App\Exports;

use Illuminate\Bus\Queueable;
use Illuminate\Database\Connection;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Queue\SerializesModels;
use Maatwebsite\Excel\Concerns\Exportable;
use Maatwebsite\Excel\Concerns\FromQuery;
use Illuminate\Contracts\Queue\ShouldQueue;
use Maatwebsite\Excel\Concerns\WithHeadings;
use Maatwebsite\Excel\Concerns\WithCustomQuerySize;

class DataTablesExport implements FromQuery, WithCustomQuerySize, WithHeadings, ShouldQueue
{
    use Queueable, SerializesModels, Exportable, InteractsWithQueue;

    /**
     * @var Builder
     */
    private $query;
    private $columns;

    /**
     * DataTablesExport constructor.
     * @param Builder $query
     * @param array $columns
     */
    public function __construct(Builder $query, array $columns)
    {
        $this->query = $query;
        $this->columns = $columns;
    }


    /**
     * @inheritDoc
     */
    public function query()
    {
        return $this->query;
    }

    /**
     * @inheritDoc
     */
    public function querySize(): int
    {
        return $this->query->count();
    }


    public function __sleep()
    {
        return []; //Pass the names of the variables that should be serialised here
    }

    public function __wakeup()
    {
        //Since we can't serialize the connection we need to re-open it when we unserialise
        $this->connection = app(Connection::class);
    }


    /**
     * @return array
     */
    public function headings(): array
    {
        return $this->columns;
    }
}

Queue driver: Redis

Exception message from laravel failed_jobs table (Symfony\Component\Debug\Exception\FatalThrowableError: Return value of App\Exports\DataTablesExport::headings() must be of the type array, null returned in DataTablesExport.php:72)

This implementation works if I removed the headings method from DataTableExport Class

Any suggestions to add headers to the excel file i am trying to export?