When using the distance prop on a SortableContainer in a nested component an error is thrown

When using the distance property on a nested SortableContainer an exception is thrown because the active property on the manager is undefined. There is a function called getActive on the Manager class and if you do a check for the active property the error appears to be fixed.
```javascript
getActive() {
if(!this.active)
return null;

return find(this.refs[this.active.collection], ({node}) => node.sortableInfo.index == this.active.index);
}
```

I am unsure of the specifics of the library so do not really feel confident in issuing a pull request for this there is probably a more elegant fix. This error only occurs when using the distance property though.

Sandy.

Start with the Manager class and its getActive() method, then reproduce a nested SortableContainer using the distance property. Confirm the active-manager state that triggers the exception and verify that the scenario no longer throws without breaking active-item lookup.

Written by the indexing model from the issue text.