ExcelDna use ExcelDnaUtil.Application in other class report error System.InvalidOperationException

Question

I hava a UDFGenerator class and a UDFtest class,in UDFGenerator I created a static variable based on ExcelDnaUtil.Application, and use it in a static function, if that all in the UDFGenerator it's worked ,but when I call UDFGenerator's function in UDFtest , it's wrong.

 public static class UDFGenerator
    {
        public static readonly Excel.Application ExcelApp = ExcelDnaUtil.Application as Excel.Application;
public static async Task<object[,]> GetApiResult(string id, string ApiType,string paramListStr, object[] paramList){
...
}
}

 public static class UDFtest
    {
        public static object GETROOT(object p0, object p1)
        {
            return ExcelAsyncUtil.Run("GETROOT", new object[] { p0, p1 }, delegate
            {
                return UDFGenerator.GetApiResult("001659ca6e3ed001d9a9be59d903a15c", "2", "NM,ENM", new object[] { p0, p1 }).GetAwaiter().GetResult();
            });
        }
}

How could I fix it?