我有以下代碼設(shè)置 onLoad 以在關(guān)聯(lián)帳戶標(biāo)記為“服務(wù)監(jiān)視”時在“發(fā)貨”記錄上生成橫幅。該代碼當(dāng)前可以運行，但它會生成錯誤警報"unable to get property '0' of undefined or null reference"。當(dāng)用戶創(chuàng)建新的貨件記錄時會發(fā)生此錯誤，因為帳戶字段尚無值。如何配置代碼以忽略帳戶字段中的 NULL 值？function checkServiceWatch() {    try{        var account = Xrm.Page.getAttribute("cmm_account").getValue();        var accountid = account[0].id;        var formattedGuid = accountid.replace("}", "");        accountid = formattedGuid.replace("{", "");        // alert("Accountid: " + accountid);  // does that ID have brackets around it?        // alert("Request: " + Xrm.Page.context.getClientUrl() + "/api/data/v8.2/accounts(" + accountid + ")?$select=cmm_servicewatch");        var req = new XMLHttpRequest();        req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/accounts(" + accountid + ")?$select=cmm_servicewatch", true);        req.setRequestHeader("OData-MaxVersion", "4.0");        req.setRequestHeader("OData-Version", "4.0");        req.setRequestHeader("Accept", "application/json");        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");        req.onreadystatechange = function()應(yīng)忽略正在創(chuàng)建的記錄，但會在具有帳戶值的現(xiàn)有貨件上生成橫幅。