Update ChannelHelper.cs (#606)

* Update ChannelHelper.cs

* typo

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -91,6 +91,8 @@ namespace Discord.Rest
             var guildId = (channel as IGuildChannel)?.GuildId;
             var guild = guildId != null ? await (client as IDiscordClient).GetGuildAsync(guildId.Value, CacheMode.CacheOnly).ConfigureAwait(false) : null;
             var model = await client.ApiClient.GetChannelMessageAsync(channel.Id, id, options).ConfigureAwait(false);
+            if (model == null)
+                return null;
             var author = GetAuthor(client, guild, model.Author.Value, model.WebhookId.ToNullable());
             return RestMessage.Create(client, channel, author, model);
         }