Add comment about roomId potentially not being resolved following an update from RoomViewStore

src/components/structures/RoomView.js

@@ -183,6 +183,8 @@ module.exports = React.createClass({
         }
 
         this.setState(newState, () => {
+            // At this point, this.state.roomId could be null (e.g. the alias might not
+            // have been resolved yet) so anything called here must handle this case.
             this._onHaveRoom();
             this.onRoom(MatrixClientPeg.get().getRoom(this.state.roomId));
         });