Php Laravel excel导出标题（）仅在队列中引发异常

我正在尝试使用导出大型数据。如果我从导出类中删除Headers方法，效果会很好。如果我试图包括标题方法，它会给我一个例外

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

在我的控制器中

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

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

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

出口类

<?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;
    }
}

---

标题（）
或
标题（）
？您是否能够
dd（$columns）
？结果是什么？是的，它是一个列数组，例如：[0=>'name'，1=>'age']，所以在没有队列的情况下，标题可以吗？是的，它工作正常