Custom Ribbon Delete Dialog on sub grid - Retrieving to see if data exist. Click Cancel then retrieve again get error message

var subGridName = null;
var selectedGridControl = null;
var subGridEntityName = null;
var entityTypeCode = null;
var selectedEntityReferences = [];
var originalFunctionDeleteRecordSubgrid = null;

function ribbonSubgridDelete(entity_subGrid) {

    var caseId = Xrm.Page.data.entity.getId().replace(/{/g, "").replace(/}/g, "");

    var entitysubgridtype = entity_subGrid.split('-');
    subGridEntityName = entitysubgridtype[0];
    subGridName = entitysubgridtype[1];
    selectedGridControl = subGridName;

    var selectedRows = Xrm.Page.getControl(subGridName).getGrid().getSelectedRows();
    selectedEntityReferences = [];
    selectedRows.forEach(function (selectedRow, i) {
        selectedEntityReferences.push(selectedRow.getData().getEntity().getEntityReference());
    });

    var selectRowRecordId = selectedEntityReferences[0].id.replace(/{/g, "").replace(/}/g, "");

    entityTypeCode = "1222";
    try {
        //to store the original function ones
        originalFunctionDeleteRecordSubgrid = Mscrm.GridCommandActions.deleteRecords;

        //add new standard subgrid
        Mscrm.GridCommandActions.deleteRecords = function (selectedGridControl, selectedEntityReferences, entityTypeCode) {
            getCheckifDataExist(caseId, selectRowRecordId);
        }
    }
    catch (e) {

    }
}


function getCheckifDataExist(caseId, selectRowRecordId) {
    var iselect = "$select=new_name";
    var ifilter = "&$filter=(new_selectId/Id eq guid'" + selectRowRecordId + "' and new_CaseId/Id eq guid'" + caseId + "')";

    var options = iselect + ifilter;

    //Same error message using this API
    //SDK.REST.retrieveMultipleRecords(subGridEntityName, options, checkIfDataExistCallBack, checkIfDataExistError, checkIfDataExistComplete);


    $.ajax({
        type: "GET",
        contentType: "application/json; charset=utf-8",
        datatype: "json",
        url: Xrm.Page.context.getClientUrl() + "/api/data/v8.1/new_EntityName?$select=_new_caseid_value,_new_selectid_value,new_name&$filter=_new_caseid_value eq " + caseId + " and  _new_selectid_value eq " + selectRowRecordId,
        beforeSend: function (XMLHttpRequest) {
            XMLHttpRequest.setRequestHeader("OData-MaxVersion", "4.0");
            XMLHttpRequest.setRequestHeader("OData-Version", "4.0");
            XMLHttpRequest.setRequestHeader("Accept", "application/json");
            XMLHttpRequest.setRequestHeader("Prefer", "odata.include-annotations=\"*\",odata.maxpagesize=1");
        },
        async: true,
        success: function (data, textStatus, xhr) {
            var results = data;
               if (results.length > 0) {
                alert("This record has already been assigned to a case");
                return false;
            } else {
                originalFunctionDeleteRecordSubgrid(subGridName, selectedEntityReferences[0], entityTypeCode);
                return true;
            }

        },
        error: function (xhr, textStatus, errorThrown) {
            Xrm.Utility.alertDialog(textStatus + " " + errorThrown);
        }
    });
}