Accessing undefined this._input element on Firefox v52 and v60

This is a followup ticket to GHS-135325.

To reproduce the problem :

1. Install 7.6.9 Jira (Issue is reproducible in both standalone server as well as Data center)
2. Have checklist plugin ( checklist-4.0.3.jar ) installed.
3. Create a issue in a SCRUM Software project . Create a Sample Checklist custom Field and have it on the issue screen.
4. Install Scriptrunner add-on  (groovyrunner-5.4.39.jar) or higher
5. Go to the Projects --> Click on the issue which has the custom field from checklist. 

Error will occur in JavaScript console.

This problem is caused by us listening to DOMNodeInserted events. We should use MutationObserver API instead.