TypeError: $container.isotope is not a function (WordPress & Isotope)

I have a problem with isotope & wordpress. Im using an easy script which is not working when integrated into wordpress. I get the following error:

TypeError: $container.isotope is not a function”. This is the code:

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jQuery(function($) {
    /* ISOTOPE */
    var $container = $('.flex-grid');
    $container.isotope({
        itemSelector: '.grid-item',
        percentPosition: true,
        isAnimated: false,
        transitionDuration: 0,
        masonry: {
            columnWidth: '.grid-sizer',
        }
    });
});

The script is loaded in the side footer as the last script.