PLT-1909 Fixed error rendering pending posts (#3041)

* Fixed error caused by checking if a post is a system message

* Fixed trying to edit system posts when using the up arrow hotkey

webapp/components/rhs_comment.jsx

@@ -85,7 +85,7 @@ export default class RhsComment extends React.Component {
 
         const isOwner = this.props.currentUser.id === post.user_id;
         var isAdmin = TeamStore.isTeamAdminForCurrentTeam() || UserStore.isSystemAdminForCurrentUser();
-        const isSystemMessage = post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
+        const isSystemMessage = post.type && post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
 
         var dropdownContents = [];