[Commands] Autocomplete provider throws null reference if command has user parameter

[BlackLotusLV]

Summary

If a slash command has a parameter DiscordUser and is followed by a auto complete provider, whenever you select to fill out the auto fill field on discord, it throws an null reference error.

What version of the library are you using?

v5.0.0-nightly (make sure you are using the latest nightly!)

What .NET version are you using? Make sure to use the latest patch release for your major version.

.NET 8.0

Operating System

Win11

Reproduction Steps

Make a slash command that contains a DiscordUser followed by an auto complete provider
example

[Command("unwarn"), Description("Removes a warning from the user"), RequirePermissions(Permissions.KickMembers)]
public async Task UnWarn(SlashCommandContext ctx,
    [Description("User to remove the warning for")] DiscordUser user,
    [Description("The ID of a specific warning. Leave as is if don't want a specific one"), SlashAutoCompleteProvider(typeof(ActiveWarningAutocompleteProvider))] long warningId = -1)

On discord, try to fill out the command, when you try to input the autocomplete field, it will throw an error

Trace Logs

[2024:02:25 13:26:23 ERR] [202|CMDError    ] blacklotus(86725763428028416) tried executing 'Mod unwarn' command, but it errored
DSharpPlus.Commands.Exceptions.ArgumentParseException: Failed to parse user.
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at DSharpPlus.Commands.Converters.DiscordUserConverter.ConvertAsync(ConverterContext context, InteractionCreateEventArgs eventArgs)
   at DSharpPlus.Commands.Processors.BaseCommandProcessor`4.ExecuteConverterAsync[T](TConverter converter, TConverterContext converterContext, TEventArgs eventArgs)
   at DSharpPlus.Commands.Processors.SlashCommands.SlashCommandProcessor.ParseAutoCompleteArgumentsAsync(InteractionConverterContext converterContext, InteractionCreateEventArgs eventArgs)
   --- End of inner exception stack trace ---

Exceptions or other error messages

No response

Anything else you'd like to share

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