JsonSerializer.Deserialize results in a null ref when attempting to deserialize open generic Nullable<>

[pranavkm]

Not a useful scenario by any means, but the exception message could be better:

var type = typeof(Nullable<>);
JsonSerializer.Deserialize("1", type);

Unhandled exception. System.ArgumentNullException: Value cannot be null. (Parameter 'key')
   at System.Collections.Concurrent.ConcurrentDictionary`2.ThrowKeyNullException()
   at System.Collections.Concurrent.ConcurrentDictionary`2.TryGetValue(TKey key, TValue& value)
   at System.Text.Json.JsonSerializerOptions.GetConverter(Type typeToConvert)
   at System.Text.Json.JsonClassInfo.GetClassType(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type classType)
   at System.Text.Json.JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
   at System.Text.Json.JsonSerializer.ParseCore(String json, Type returnType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Deserialize(String json, Type returnType, JsonSerializerOptions options)

[pranavkm]

In a similar vein, for other open generic types, the exception message is a bit obtuse:

static void Main(string[] args)
{
    var type = typeof(Test<>);
    JsonSerializer.Deserialize("1", type);
}

class Test<TValue>
{
}

Unhandled exception. System.ArgumentException: Cannot create an instance of System.Text.Json.JsonPropertyInfoNotNullable`4[ConsoleApp17.Program+Test`1[TValue],TValue,TValue,TValue] because Type.ContainsGenericParameters is true.
   at System.RuntimeType.CreateInstanceCheckThis()
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.Text.Json.JsonClassInfo.CreateProperty(Type declaredPropertyType, Type runtimePropertyType, Type implementedPropertyType, PropertyInfo propertyInfo, Type parentClassType, JsonConverter converter, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo.AddProperty(Type propertyType, PropertyInfo propertyInfo, Type classType, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type classType)
   at System.Text.Json.JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
   at System.Text.Json.JsonSerializer.ParseCore(String json, Type returnType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Deserialize(String json, Type returnType, JsonSerializerOptions options)

Could the serializer check further up the stack if the type is an open generic and throw instead?

[ahsonkhan]

cc @steveharter, @layomia