Fix Finder javascript error

#### Fixes an error on Admin side for Finder #5018

[Bug report here](https://github.com/joomla/joomla-cms/issues/5018)

#### Test instructions

**Ensure you have content (ideally sample data)**

1. Components -> Smart Search -> Search filters.
2. Click on New to create a new filter.

Check your browsers console for js error

3. Apply this patch

Re check your browser
Check functionality

### Side note

##### Finder is totally dependent on mootools and  this has to change somehow...