Discord.js | TypeError:无法读取未定义的属性“”toLowerCase“”

let str = someVariable;
if (str !== undefined && str !== null) {
    let lowerCaseStr = str.toLowerCase();
    console.log(lowerCaseStr);
} else {
    console.error("Variable is undefined or null");
}

let lowerCaseStr = someObject?.someProperty?.toLowerCase();
if (lowerCaseStr !== undefined) {
    console.log(lowerCaseStr);
} else {
    console.error("Property is undefined");
}

async function fetchData() {
    try {
        let data = await someAsyncFunction();
        if (data !== undefined && data !== null) {
            let lowerCaseStr = data.toLowerCase();
            console.log(lowerCaseStr);
        } else {
            console.error("Data is undefined or null");
        }
    } catch (error) {
        console.error("Error fetching data:", error);
    }
}

client.on('message', message => {
    let content = message.content;
    if (content !== undefined && content !== null) {
        let lowerCaseContent = content.toLowerCase();
        console.log(lowerCaseContent);
    } else {
        console.error("Message content is undefined or null");
    }
});