c# 变量引发了类型“System.NullReferenceException”的异常【未解】

2024/11/14 求助，为什么“targetRoundId1”引发了类型“System.NullReferenceException”的异常

代码：

Dictionary<string, Dictionary<string,string>> exchangeRateDict1 = new ();
exchangeRateDict1.Add("USD", new Dictionary<string, string> { { "USD", "1" } });
exchangeRateDict1.Add("CNY", new Dictionary<string, string> { { "CNY", "6.5" } });
exchangeRateDict1.Add("JPY", new Dictionary<string, string> { { "JPY", "110" } });
exchangeRateDict1.Add("EUR", new Dictionary<string, string> { { "EUR", "0.8" } });

if (exchangeRateDict1.TryGetValue("USD", out var t1))
{
    var targetRoundId1 = "USD";
    var tt = t1.Where(i => i.Key == targetRoundId1).FirstOrDefault();
}
if (exchangeRateDict1.TryGetValue("CNY", out var t2))
{
    var targetRoundId1 = "CNY";
    var tt = t2.Where(i => i.Key == targetRoundId1).FirstOrDefault();
}