Fixed being unable to make the first post in a new channel (#4744)

webapp/components/post_view/post_view_controller.jsx

@@ -191,8 +191,19 @@ export default class PostViewController extends React.Component {
     onPostsViewJumpRequest(type, postId) {
         switch (type) {
         case Constants.PostsViewJumpTypes.BOTTOM: {
+            let lastViewedBottom;
             const lastPost = PostStore.getLatestPost(this.state.channel.id);
-            this.setState({scrollType: ScrollTypes.BOTTOM, lastViewedBottom: lastPost.create_at || new Date().getTime()});
+
+            if (lastPost && lastPost.create_at) {
+                lastViewedBottom = lastPost.create_at;
+            } else {
+                lastViewedBottom = new Date().getTime();
+            }
+
+            this.setState({
+                scrollType: ScrollTypes.BOTTOM,
+                lastViewedBottom
+            });
             break;
         }
         case Constants.PostsViewJumpTypes.POST: