Fixed websocket message mention detection

src/Discord.Net.Core/Utils/MentionsHelper.cs

@@ -62,6 +62,9 @@ namespace Discord
         internal static ImmutableArray<TRole> GetRoleMentions<TRole>(string text, IGuild guild)
             where TRole : class, IRole
         {
+            if (guild == null)
+                return ImmutableArray.Create<TRole>();
+
             var matches = _roleRegex.Matches(text);
             var builder = ImmutableArray.CreateBuilder<TRole>(matches.Count);
             foreach (var match in matches.OfType<Match>())

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -96,10 +96,11 @@ namespace Discord.WebSocket
             if (model.Content.IsSpecified)
             {
                 var text = model.Content.Value;
+                var guild = (Channel as SocketGuildChannel)?.Guild;
 
-                _mentionedUsers = MentionsHelper.GetUserMentions(text, null, mentions);
-                _mentionedChannelIds = MentionsHelper.GetChannelMentions(text, null);
-                _mentionedRoles = MentionsHelper.GetRoleMentions<RestRole>(text, null);
+                _mentionedUsers = MentionsHelper.GetUserMentions(text, Channel, mentions);
+                _mentionedChannelIds = MentionsHelper.GetChannelMentions(text, guild);
+                _mentionedRoles = MentionsHelper.GetRoleMentions<RestRole>(text, guild);
                 model.Content = text;
             }
         }