Fix some JS errors (#6811)

webapp/stores/post_store.jsx

@@ -71,7 +71,7 @@ class PostStoreClass extends EventEmitter {
     }
 
     getLatestReplyablePost(channelId) {
-        const postIds = getState().entities.posts.postsInChannel[channelId];
+        const postIds = getState().entities.posts.postsInChannel[channelId] || [];
         const posts = getState().entities.posts.posts;
 
         for (const postId of postIds) {

webapp/yarn.lock

@@ -5043,7 +5043,7 @@ math-expression-evaluator@^1.2.14:
 
 mattermost-redux@mattermost/mattermost-redux#webapp-master:
   version "0.0.1"
-  resolved "https://codeload.github.com/mattermost/mattermost-redux/tar.gz/435f9e5ae8a4d021892465d354b6cd331e2766d9"
+  resolved "https://codeload.github.com/mattermost/mattermost-redux/tar.gz/36bdc456028c19605263958ecb2e5b9612b4a67f"
   dependencies:
     deep-equal "1.0.1"
     harmony-reflect "1.5.1"