It looks like you're trying to initialize a datepicker when there's no #datepicker element in DOM yet (the corresponding input follows <script> element, not vice versa).
As $('#datepicker') will still return a jQuery-wrapped object (not null!), no error will be thrown.
To solve this, either make this call when DOM is loaded:
$(function() {
$('#datepicker').datepicker();
});
or move this script into very end of <body> element.
I'd choose the first option; moving all the DOM-ready actions into the same $(function() { ... }); wrapper as well.
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