fix(TextBasedChannel): added missing lastMessage functionality in textchannels (#2999)

src/client/actions/MessageCreate.js

@@ -16,7 +16,6 @@ class MessageCreateAction extends Action {
         }
         const lastMessage = messages[messages.length - 1];
         channel.lastMessageID = lastMessage.id;
-        channel.lastMessage = lastMessage;
         if (user) {
           user.lastMessageID = lastMessage.id;
           user.lastMessage = lastMessage;
@@ -31,7 +30,6 @@ class MessageCreateAction extends Action {
       } else {
         const message = channel._cacheMessage(new Message(channel, data, client));
         channel.lastMessageID = data.id;
-        channel.lastMessage = message;
         if (user) {
           user.lastMessageID = data.id;
           user.lastMessage = message;

src/structures/interfaces/TextBasedChannel.js

@@ -395,6 +395,15 @@ class TextBasedChannel {
     return 0;
   }
 
+  /**
+   * The Message object of the last message in the channel, if one was sent
+   * @type {?Message}
+   * @readonly
+   */
+  get lastMessage() {
+    return this.messages.get(this.lastMessageID) || null;
+  }
+
   /**
    * The date when the last pinned message was pinned, if there was one
    * @type {?Date}
@@ -599,6 +608,7 @@ exports.applyToClass = (structure, full = false, ignore = []) => {
       'fetchMessages',
       'fetchMessage',
       'search',
+      'lastMessage',
       'lastPinAt',
       'bulkDelete',
       'startTyping',