queue.isReady() throws error if client not initialized

Created by: gabegorelick

Description

If queue._initializing is not set, queue.isReady()will throw a TypeError: Cannot read property 'then' of undefined at https://github.com/OptimalBits/bull/blob/c0ee6be03c6d47d60465d2bae0d2566d5bf0d661/lib/queue.js#L478

_initializing is set when the main Redis client is initialized: https://github.com/OptimalBits/bull/blob/c0ee6be03c6d47d60465d2bae0d2566d5bf0d661/lib/queue.js#L150

Setting skipVersionCheck: true is the only way to trigger this as far as I can tell. Otherwise, getRedisVersion(this.client) will cause _initializing to be set before the Queue constructor returns.

Minimal, Working Test code to reproduce the issue.

const queue = new Queue('foo', {skipVersionCheck: true});
queue.isReady();

Bull version

develop

Additional information

Anything that calls isReady will also throw, e.g. queue.close(), queue.add(), etc.