Cannot read property 'error' of null occurs almost on every click

Hello,

I'm creating a custom plugin for my Wordpress site using datatables. So far, it's okay but once I click on sort, end of page(pagination), etc., I always encounter this error:

Uncaught TypeError: Cannot read property 'error' of null
at Object.success (jquery.dataTables.js?ver=4.7.5:3899)
at i (jquery.js?ver=1.12.4:2)
at Object.fireWith [as resolveWith] (jquery.js?ver=1.12.4:2)
at y (jquery.js?ver=1.12.4:4)
at XMLHttpRequest.c (jquery.js?ver=1.12.4:4)

And when I track jquery.dataTables.js?ver=4.7.5:3899, this is where the error is (line 3896):

var baseAjax = {
    "data": data,
    "success": function (json) {
    var error = json.error || json.sError;
        if ( error ) {
            _fnLog( oSettings, 0, error );
        }
            oSettings.json = json;
            callback( json );
        },
        "dataType": "json",
        "cache": false,
        "type": oSettings.sServerMethod,
        "error": function (xhr, error, thrown) {
            var ret = _fnCallbackFire( oSettings, null, 'xhr', [oSettings, null, oSettings.jqXHR] );

            if ( $.inArray( true, ret ) === -1 ) {
                if ( error == "parsererror" ) {
                    _fnLog( oSettings, 0, 'Invalid JSON response', 1 );
                }
                else if ( xhr.readyState === 4 ) {
                    _fnLog( oSettings, 0, 'Ajax error', 7 );
                }
            }

            _fnProcessingDisplay( oSettings, false );
        }
    };

What could be the issue?

Any help is highly appreciated. Thanks!

-Eli