Fixed nullref in channel mention resolving for DM channels

src/Discord.Net/Rest/Entities/Messages/Message.cs

@@ -1,5 +1,4 @@
 using Discord.API.Rest;
-using Discord.Rest;
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;

src/Discord.Net/Utilities/MentionUtils.cs

@@ -208,8 +208,6 @@ namespace Discord
         {
             if (mode == ChannelMentionHandling.Ignore) return text;
 
-            if (guild.IsAttached) //It's too expensive to do a channel lookup in REST mode
-            {
             return _channelRegex.Replace(text, new MatchEvaluator(e =>
             {
                 ulong id;
@@ -220,19 +218,21 @@ namespace Discord
                         case ChannelMentionHandling.Remove:
                             return "";
                         case ChannelMentionHandling.Name:
+                            if (guild != null && guild.IsAttached) //It's too expensive to do a channel lookup in REST mode
+                            {
                                 IGuildChannel channel = null;
-                                channel = guild?.GetChannel(id);
+                                channel = guild.GetChannel(id);
                                 if (channel != null)
                                     return $"#{channel.Name}";
                                 else
                                     return $"#deleted-channel";
                             }
+                            break;
+                    }
                 }
                 return e.Value;
             }));
         }
-            return text;
-        }
         internal static string ResolveRoleMentions(string text, IReadOnlyCollection<IRole> mentions, RoleMentionHandling mode)
         {
             if (mode == RoleMentionHandling.Ignore) return text;