Unable to set collListItem or nullreference

i am trying to get the items betwen a time period, i get error unable to set collListItem or null reference? i have 3 items in the list

function checkFreebookingdateTime() {
try {
    var listItemCustom = document.getElementById('datepicker').value;
    var listItemFromTime = document.getElementById('timepicker').value;
    var listItemtoDate = document.getElementById('datepickerto').value;
    var listItemToTime = document.getElementById('timepickerTo').value;
    var result = listItemCustom + "T" + listItemFromTime + ":00" + "Z";
    var result2 = listItemtoDate + "T" + listItemToTime + ":00" + "Z";


    getListCalender = document.getElementById('CalenderListDrop').value;
    spHostUrl = decodeURIComponent(getQueryStringParameter('SPHostUrl'));
    context = new SP.ClientContext.get_current();
    parentContext = new SP.AppContextSite(context, spHostUrl);
    web = parentContext.get_web();
    list = web.get_lists().getByTitle(getListCalender);
    context.load(list);
    var fromTime = new Date(result);
    var fromTimeConverted = fromTime.toJSON().split('T')[0] + "T08:00:00Z";
    var toTime = new Date(result2);
    var toTimeConverted = toTime.toJSON().split('T')[0] + "T17:00:00Z";


    var camlQuery = new SP.CamlQuery();
    camlQuery.set_viewXml("<View><Query><Where><And><Geq><FieldRef Name=\'EventDate\' /><Value IncludeTimeValue=\'TRUE\' Type=\'DateTime\'>'+ fromTimeConverted +'</Value></Geq><Leq><FieldRef Name=\'EndDate\' /><Value IncludeTimeValue=\'TRUE\' Type=\'DateTime\'>'+ toTimeConverted +'</Value></Leq></And></Where></Query></View>");


    this.collListItem = list.getItems(camlQuery);
    context.load(collListItem);
    context.executeQueryAsync(
        function () {


            var itemCount = collListItem.get_count();
            if (itemCount > 0) {


                alert(itemCount);

                var listItemEnumerator = collListItem.getEnumerator();
                while (listItemEnumerator.moveNext()) {
                var oListItem = listItemEnumerator.get_current();
                var array = new Array(oListItem.get_item('EventDate').toLocaleString() + "   " + oListItem.get_item('EndDate').toLocaleString());
                for (var i = 0; i < array.length; i++) {
                        var date = array[i];
                        alert(date);
                    }

                }
            }


        },
        function() {
        }
    );

  } catch (e) {
    alert("Fel");
  }
 }