Possible null reference argument for parameter 'value' in 'T? JsonConvert.DeserializeObject(string value)'

Question

Trying to learn .NET Core WebAPI and using Redis. Found this example app and tried. Have the same code but I'm getting error below.

PS: Please tell if I need to add Program.cs too.

 System.ArgumentNullException: Value cannot be null. (Parameter 'configuration')
   at StackExchange.Redis.ConfigurationOptions.DoParse(String configuration, Boolean ignoreUnknown) in /_/src/StackExchange.Redis/ConfigurationOptions.cs:line 752
   at StackExchange.Redis.ConfigurationOptions.Parse(String configuration, Boolean ignoreUnknown) in /_/src/StackExchange.Redis/ConfigurationOptions.cs:line 582
   at StackExchange.Redis.ConfigurationOptions.Parse(String configuration) in /_/src/StackExchange.Redis/ConfigurationOptions.cs:line 572
   at StackExchange.Redis.ConnectionMultiplexer.Connect(String configuration, TextWriter log) in /_/src/StackExchange.Redis/ConnectionMultiplexer.cs:line 655
   at ApiProject.Cache.ConnectionHelper.<>c.<.cctor>b__0_0() in C:\Users\Mustafa\source\repos\ApiProject\Cache\ConnectionHelper.cs:line 10
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)
--- End of stack trace from previous location ---
   at System.Lazy`1.CreateValue()
   at ApiProject.Cache.ConnectionHelper.get_Connection() in C:\Users\Mustafa\source\repos\ApiProject\Cache\ConnectionHelper.cs:line 18
   at ApiProject.Cache.CacheService.ConfigureRedis() in C:\Users\Mustafa\source\repos\ApiProject\Cache\CacheService.cs:line 14
   at ApiProject.Cache.CacheService..ctor() in C:\Users\Mustafa\source\repos\ApiProject\Cache\CacheService.cs:line 10
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Span`1& arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass2_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, Boolean isDefaultParameterRequired)
   at lambda_method16(Closure , IServiceProvider , Object[] )
   at Microsoft.AspNetCore.Mvc.Controllers.ControllerActivatorProvider.<>c__DisplayClass7_0.<CreateActivator>b__0(ControllerContext controllerContext)
   at Microsoft.AspNetCore.Mvc.Controllers.ControllerFactoryProvider.<>c__DisplayClass6_0.<CreateControllerFactory>g__CreateController|0(ControllerContext controllerContext)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.Policy.AuthorizationMiddlewareResultHandler.HandleAsync(RequestDelegate next, HttpContext context, AuthorizationPolicy policy, PolicyAuthorizationResult authorizeResult)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

ConnectionHelper class:

public class ConnectionHelper
    {
        static ConnectionHelper()
        {
            ConnectionHelper.lazyConnection = new Lazy<ConnectionMultiplexer>(() =>
            {
                return ConnectionMultiplexer.Connect(ConfigurationManager.AppSetting["RedisURL"]);
            });
        }
        private static Lazy<ConnectionMultiplexer> lazyConnection;
        public static ConnectionMultiplexer Connection
        {
            get
            {
                return lazyConnection.Value;
            }
        }
    }

CacheSerevice class: JsonConvert.DeserializeObject<T>(value); returns null.

public class CacheService : ICacheService
    {
        private IDatabase _db;
        public CacheService()
        {
            ConfigureRedis();
        }
        private void ConfigureRedis()
        {
            _db = ConnectionHelper.Connection.GetDatabase();
        }
        public T GetData<T>(string? key)
        {
            var value = _db.StringGet(key);
            if (!string.IsNullOrEmpty(value))
            {
                return JsonConvert.DeserializeObject<T>(value);
            }
            return default;
        }
        public bool SetData<T>(string key, T value, DateTimeOffset expirationTime)
        {
            TimeSpan expiryTime = expirationTime.DateTime.Subtract(DateTime.Now);
            var isSet = _db.StringSet(key, JsonConvert.SerializeObject(value), expiryTime);
            return isSet;
        }
        public object RemoveData(string key)
        {
            bool _isKeyExist = _db.KeyExists(key);
            if (_isKeyExist == true)
            {
                return _db.KeyDelete(key);
            }
            return false;
        }
}

appsettings.json:

{
  "RedisURL": "127.0.0.1:6379",
  "JWT": {
    "ValidAudience": "http://localhost:7184",
    "ValidIssuer": "http://localhost:7184",
    "Secret": "JWTAuthentication@777"
  },

  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "ConnectionStrings": {
    "DefaultConnection": "Data Source=.;Initial Catalog=ApiTest_Database;Integrated Security=True;MultipleActiveResultSets=True;"
  }
}