Fix NullReferenceException caused by Options being undefined. (#2549)

src/Discord.Net.Core/Entities/Interactions/SlashCommands/SlashCommandBuilder.cs

@@ -664,7 +664,9 @@ namespace Discord
             if (options == null)
                 throw new ArgumentNullException(nameof(options), "Options cannot be null!");
 
-            if ((Options?.Count ?? 0) + options.Length > SlashCommandBuilder.MaxOptionsCount)
+            Options ??= new List<SlashCommandOptionBuilder>();
+
+            if (Options.Count + options.Length > SlashCommandBuilder.MaxOptionsCount)
                 throw new ArgumentOutOfRangeException(nameof(options), $"There can only be {SlashCommandBuilder.MaxOptionsCount} options per sub command group!");
 
             foreach (var option in options)

test/Discord.Net.Tests.Unit/CommandBuilderTests.cs

@@ -17,7 +17,7 @@ public class CommandBuilderTests
             ApplicationCommandOptionType.String,
             "option1 description",
             isRequired: true,
-            choices: new [] 
+            choices: new []
             {
                 new ApplicationCommandOptionChoiceProperties()
                 {
@@ -34,4 +34,22 @@ public class CommandBuilderTests
             .AddChoice("choice2", "2"));
         command.Build();
     }
+
+    [Fact]
+    public void BuildSubSlashCommand()
+    {
+        var command = new SlashCommandBuilder()
+            .WithName("command").WithDescription("Command desc.")
+            .AddOptions(new SlashCommandOptionBuilder()
+                .WithType(ApplicationCommandOptionType.SubCommand)
+                .WithName("subcommand").WithDescription("Subcommand desc.")
+                .AddOptions(
+                    new SlashCommandOptionBuilder()
+                        .WithType(ApplicationCommandOptionType.String)
+                        .WithName("name1").WithDescription("desc1"),
+                    new SlashCommandOptionBuilder()
+                        .WithType(ApplicationCommandOptionType.String)
+                        .WithName("name2").WithDescription("desc2")));
+        command.Build();
+    }
 }