When update title of a button node, need to get title node via setter (facebookarchive#3167)

* When update title of a button node, need to get title node via setter

* Explain why we need to call self.titleNode

* Fix previous comment

Author: nguyenhuy

Source/ASButtonNode.mm

@@ -197,13 +197,12 @@ - (void)updateTitle
     newTitle = _normalAttributedTitle;
   }
 
-  ASTextNode *titleNode = _titleNode;
-  
-  if ((titleNode != nil || newTitle.length > 0) && [titleNode.attributedText isEqualToAttributedString:newTitle] == NO) {
-    titleNode.attributedText = newTitle;
+  // Calling self.titleNode is essential here because _titleNode is lazily created by the getter.
+  if ((_titleNode != nil || newTitle.length > 0) && [self.titleNode.attributedText isEqualToAttributedString:newTitle] == NO) {
+    _titleNode.attributedText = newTitle;
     __instanceLock__.unlock();
 
-    self.accessibilityLabel = titleNode.accessibilityLabel;
+    self.accessibilityLabel = _titleNode.accessibilityLabel;
     [self setNeedsLayout];
     return;
   }