Check if event is null before stopPropagation()

Brought up in Issue #17:

Calling colpickShow() throws an error when calling stop propagation since the event it is using is null. I just added an if to check if ev is null; if it isn't, call stopPropagation(), else skip it.

```
if(ev) {
    // Prevent the trigger of any direct parent
    ev.stopPropagation();
}
```