<aura:attribute type="List" name="filterCriteriaList"/>

getOperator : function(component, event, helper){
        var fieldName = component.find("fieldSelect").get("v.value");
        var index = component.find("hiddenIndex").get("v.value");
        console.log(' >>>>> ' + JSON.stringify(component.get('v.filterCriteriaList')));
        var action = component.get("c.fetchOperators");
        action.setParams({ "index" : index, "fieldName" : fieldName, "filterCriteriaItemJson" : JSON.stringify(component.get('v.filterCriteriaList'))});

        action.setCallback(this, function(response) {

@auraEnabled
    public static void fetchOperators(Integer currentIndex, String fieldName, String filterCriteriaItemJson){
        //System.debug('***filterCriteriaItemJson='+filterCriteriaItemJson);
        List<FilterCriteria> filterCriteriaItems = (List<FilterCriteria>)JSON.deserializeStrict(filterCriteriaItemJson,List<FilterCriteria>.class);
        for(FilterCriteria f : filterCriteriaItems){
            if (f.index == currentIndex && f.fieldName.equalsIgnoreCase(fieldName)){
                f.operatorsList  = Utility.getOperator(f.mapFieldTypes.get(fieldName));
            }
        }
    }

FATAL_ERROR System.UnexpectedException: Salesforce System Error: 1803375914-142009 (577092351) (577092351)
FATAL_ERROR Class.System.JSON.deserializeStrict: line 19, column 1
 FATAL_ERROR Class.PaginationFilterController.fetchOperators: line 61, column 1