Fixed crash on leaving a group channel

src/Discord.Net/Entities/WebSocket/CachedDMChannel.cs

@@ -13,7 +13,7 @@ namespace Discord
         public new DiscordSocketClient Discord => base.Discord as DiscordSocketClient;
         public new CachedDMUser Recipient => base.Recipient as CachedDMUser;
         public IReadOnlyCollection<ICachedUser> Members => ImmutableArray.Create<ICachedUser>(Discord.CurrentUser, Recipient);
-        IReadOnlyCollection<CachedDMUser> ICachedPrivateChannel.Recipients => ImmutableArray.Create(Recipient);
+        IReadOnlyCollection<ICachedUser> ICachedPrivateChannel.Recipients => ImmutableArray.Create(Recipient);
 
         public CachedDMChannel(DiscordSocketClient discord, CachedDMUser recipient, Model model)
             : base(discord, recipient, model)

src/Discord.Net/Entities/WebSocket/CachedGlobalUser.cs

@@ -11,6 +11,7 @@ namespace Discord
         public Presence Presence { get; private set; }
 
         public new DiscordSocketClient Discord { get { throw new NotSupportedException(); } }
+        CachedGlobalUser ICachedUser.User => this;
 
         public CachedGlobalUser(Model model) 
             : base(model)

src/Discord.Net/Entities/WebSocket/CachedGroupChannel.cs

@@ -8,7 +8,6 @@ using MessageModel = Discord.API.Message;
 using Model = Discord.API.Channel;
 using UserModel = Discord.API.User;
 using VoiceStateModel = Discord.API.VoiceState;
-using Discord.API;
 
 namespace Discord
 {
@@ -20,7 +19,7 @@ namespace Discord
         public new DiscordSocketClient Discord => base.Discord as DiscordSocketClient;
         public IReadOnlyCollection<ICachedUser> Members 
             => _users.Select(x => x.Value as ICachedUser).Concat(ImmutableArray.Create(Discord.CurrentUser)).ToReadOnlyCollection(() => _users.Count + 1);
-        public new IReadOnlyCollection<CachedDMUser> Recipients => _users.Cast<CachedDMUser>().ToReadOnlyCollection(_users);
+        public new IReadOnlyCollection<ICachedUser> Recipients => _users.Select(x => x.Value as ICachedUser).ToReadOnlyCollection(_users);
 
         public CachedGroupChannel(DiscordSocketClient discord, Model model)
             : base(discord, model)

src/Discord.Net/Entities/WebSocket/CachedSelfUser.cs

@@ -1,10 +1,12 @@
-using Model = Discord.API.User;
+using System;
+using Model = Discord.API.User;
 
 namespace Discord
 {
     internal class CachedSelfUser : SelfUser, ICachedUser
     {
         public new DiscordSocketClient Discord => base.Discord as DiscordSocketClient;
+        CachedGlobalUser ICachedUser.User { get { throw new NotSupportedException(); } }
 
         public CachedSelfUser(DiscordSocketClient discord, Model model) 
             : base(discord, model)

src/Discord.Net/Entities/WebSocket/ICachedPrivateChannel.cs

@@ -4,6 +4,6 @@ namespace Discord
 {
     internal interface ICachedPrivateChannel : ICachedChannel, IPrivateChannel
     {
-        new IReadOnlyCollection<CachedDMUser> Recipients { get; }
+        new IReadOnlyCollection<ICachedUser> Recipients { get; }
     }
 }

src/Discord.Net/Entities/WebSocket/ICachedUser.cs

@@ -2,6 +2,8 @@
 {
     internal interface ICachedUser : IUser, ICachedEntity<ulong>
     {
+        CachedGlobalUser User { get; }
+
         ICachedUser Clone();
     }
 }