Unexpected warning message to my WordPress posts which group and listed out by year

The issue is here:

if($year !== $year_check){

The first time that conditional is used in the loop – $year_check has not been defined.

The solution is to simply define it before this:
<?php if( have_posts() ) : ?>

For example:

<?php $year_check = 0; ?>
<?php if( have_posts() ) : ?>

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