Null reference when registering nested modules with no Lifespan attribute

Created by: ostorc

Summary

When registering transient module with nested module without Lifespan attribute you get null reference exception.

Details

Consider this example module:

[Group("test")]
[ModuleLifespan(ModuleLifespan.Transient)]
public class Module: BaseCommandModule
{
         // ...
        [Group(name: "subgroup")]
        public class Nested : BaseCommandModule
        {
              // ...
        }
}

you will get exception at https://github.com/DSharpPlus/DSharpPlus/blob/838ffc1489b80510cc24ab62ed252c28c51fe3f0/DSharpPlus.CommandsNext/CommandsNextExtension.cs#L423

because lifespan is null at this moment.

Few lines before this the null is checked but not here https://github.com/DSharpPlus/DSharpPlus/blob/838ffc1489b80510cc24ab62ed252c28c51fe3f0/DSharpPlus.CommandsNext/CommandsNextExtension.cs#L419