Yarja Datatable Laravel - Null Date Value - laravel

Question

I am having issue with handling a null values within a datatable.
In short I have a table with outstanding fees, which has a default null value in the table and is updated the Date paid when the person makes the payment.
This my ajax request in my controller
public function getTermFees(Request $request)
{
if($request->ajax()) {
$termid = TermMapping::latest()->value('id');
$termfees = TermFees::where('term_id', '=', $termid)->get();
return DataTables::of($termfees)
->addColumn('first_name', function($termfees) {
return $termfees->member->first_name;
})
->addColumn('last_name', function($termfees) {
return $termfees->member->last_name;
})
->addColumn('membership', function($termfees) {
return $termfees->member->membership_number;
})
->addColumn('action', function($row){
$btn = '<i class="fa fa-info"></i>';
return $btn;
})
->editColumn('date_paid', function($termfees){
return (is_null($termfees->date_paid) ? "-" : date('d/m/Y', strtotime($termfees->date_paid)));
})
->make(true);
}
}
This is my view
<script>
$(function() {
var table=$('#termfee-table').DataTable({
processing: true,
serverSide: true,
pageLength: 50,
ajax: '{{ route('getTermFees') }}',
columns: [
{ data: 'membership'},
{ data: 'first_name'},
{ data: 'last_name'},
{ data: 'status'},
{ data: 'date_paid', "defaultContent": "<i>-</i>"},
{ data: 'action', orderable: false, searchable: false}
],
});
})
</script>
Currently all fields in my Date Paid are showing "-" even if there is a date value in the table.
Any help would be great.
Thanks