TypeScript Narrowing 似乎不起作用：对象可能未定义

好的，我刚开始打字，我对以下代码中的Object is possibly undefined错误感到困惑(这段代码已高度缩写):

interface IUrlData {
    wsPropertyNames: string[];
    routeValuesRegExp: RegExp;
}

export default class Builder implements IBuilder {
    private _urlData?: IUrlData;
    async build(): Promise<IConfig> {
        // VS Code Intellisense says that this._urlData is IUrlData | undefined in the next line.
        if (this._urlData) {
            // In this next line, VS Code Intellisense says this._urlData is IUrlData (no undefined, so narrowed by the if).
            this._urlData.wsPropertyNames.forEach((value) => {
                const obj = wjConfig[value];
                if (isConfig(obj)) {
                    // This is where the error happens.  All of the sudden, this._urlData is back to IUrlData | undefined.
                    makeWsUrlFunctions(obj, this._urlData.routeValuesRegExp);
                }
            });
        }
    }
}

那么，有没有人可以帮我指出我的错误，或者提供一个解决办法，如果这是由于错误或类似的错误而被打字搞砸的东西？

非常感谢.