Checking if childFooter is set before trying to expand

[chrux]

There are cases where the childFooter is null and we're getting this error. So similar to what you have on collapse we should validate first.

ERROR Error: Uncaught (in promise): TypeError: Cannot read property 'nativeElement' of null TypeError: Cannot read property 'nativeElement' of null at IonicPullupComponent.expand (http://localhost/module-es2015.js:4817:49)