FIX: safeguard in case the message is active during transition (#21235)

Prior to this fix, we could sometimes still have an active message while the DOM node was already removed.

plugins/chat/assets/javascripts/discourse/components/chat-message-actions-desktop.js

@@ -45,6 +45,10 @@ export default class ChatMessageActionsDesktop extends Component {
         this.context
       );
 
+      if (!messageContainer) {
+        return;
+      }
+
       this.popper = createPopper(messageContainer, element, {
         placement: "top-end",
         strategy: "fixed",