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我正在使用Laravel 10。我需要在这个结构中添加数据：

$json = [
    "groupCode" => $groupCode,
    "title" => "Solicitud de Firma",
    "description" => "Solicitud de Firma - Descripción",
    "customization" => [
        "requestMailSubject" => "Mi Email Subject",
        "requestMailBody" => "Mi Email Body",
        "requestSmsBody" => "SMS Body"
    ],
    "recipients" => [
        [
            "key" => "firmante01",
            "presential" => true
        ]
    ],
    "messages" => [
        [
            "document" => [
                "templateType" => "base64",
                "templateCode" => $templateCode, //"564FDSA4765",
                "templateReference" => $pdfInBase64,
                "autoFinalize" => true,
                "formRequired" => true,
                //"items" => $items,
            ],
            "policies"  => [
                "attachments" => [], // it´s filled below
            ],
            "metadataList" => $metaDataList,
            "callbackURL" => "",
            "callbackAuthorization" => "Basic " . "viafirmaapi" . ":" . "f1rm4v1420$",
            "callbackMails" => "",
            "callbackMailFilename" => "PRECONTRATO_" . $precontract["insert_id"]["precontract_id"] . ".pdf"
        ],
    ],
];

在这个数组中，我需要动态地将数据添加到：

"policies"  => [
    "attachments" => [], // it's filled below
],

为了做到这一点，我尝试：

$precontractDocument = PrecontractAttachment::where('precontract_id', $precontract["insert_id"]["precontract_id"])->get();
$attachments = [];
$document = [];
$tokens = [];
for ($i = 0; $i < count($precontractDocument); $i++) {
    array_push($tokens, $this->getSecurityTokensViaFirma());
    $attachments["type"] = "PDF";
    $attachments["helpText"] = "Documento adjunto";
    $attachments["fileReference"] = $tokens[$i];
    $attachments["fileName"] = $precontractDocument[$i]["filename_result"];
    $attachments["readOnly"] = true;
    array_push($document, $attachments);
    $this->uploadDocumentViafirma($document, $tokens, $precontract["insert_id"]["precontract_id"]);
}
// add items document into metadatalist
$json["messages"]["policies"]["attachments"] = $document;

这段代码只添加了一个文档，但是我需要将子数组数据添加到任何文档中。
我目前正在做：

// add items document into metadatalist
array_push($json["messages"]["policies"]["attachments"], $document);

但这将返回local.ERROR: array_push(): Argument #1 ($array) must be of type array, null given
我不知道我可以做些什么来添加数据：

"messages" => [
    [
        "policies"  => [
            "attachments" => [], // it´s filled below
        ],
    ]
]