To prevent accessing properties of null/undefined object

I made some change to prevent accessing properties of null/undefined object.

Every time my firebug will catch this.container is null when loading script. So I changed like this

```javascript
         if (!this.container){
           if(doc.body) this.container = doc.body;
           else throw 'document.body is null'
         }
         this.container.appendChild(el)
```

And I'll trigger a remove() before log() to prevent stacking message. But the first time I trigger remove() . It will also triggered error. So I changed like this

```javascript
if (cb && this.currentMsg) this.currentMsg.cb = cb
```