Undefined offset

I am trying to adjust WordPress core Archives Widget so it appears as a nice looking drop down menu. I did it and it works, but when wp_debug is set to true it shows the following Notice: Undefined offset: 1 in C:\xampp…

it is in this code:

<?php
		/**
		 * Filter the arguments for the Archives widget.
		 *
		 * @since 2.8.0
		 *
		 * @see wp_get_archives()
		 *
		 * @param array $args An array of Archives option arguments.
		 */
		$archives = strip_tags(wp_get_archives( apply_filters( 'nsgen_archive_widgets_args', array(
			'type'            => 'monthly',
			'format'          => 'custom',
			'echo'            => 0,
			'after'           => ','
		) ) ) );
		$archives = explode(',', $archives);
		$months = array();
		$years = array();
		// Grab our years first
		foreach ($archives as $archive) {
			$archive = explode(' ', $archive);
			if (isset($archive[1])) {
				array_push($years, $archive[1]);
			}
		}
		$years = array_values(array_unique($years));
		$i = 0;
		foreach ($years as $year) {
		?><li class="nsgen-archive-year"><a><?php
			echo $year;
			?></a><ul><?php
			foreach ($archives as $archive) {
				$archive = explode(' ', $archive);
                                   if ($archive[1] == $year) {
					echo '<li class="nsgen-archive-month"><a href="' .
					// Get the archive link
					get_month_link($year, date("m", strtotime($archive[0] . '-' . $year))) .
					'">' . trim($archive[0]) . '</a></li>';
      			}
			}
			?></ul><?php
		?></li><?php
		}
?>

this part: if ($archive[1] == $year) {
Can some one help, please.