PHP Warning: Attempt to read property “post_content” on null

Recently I upgraded my PHP version from 7.4 to 8.1.

Now I’m getting error_log as given below:

PHP Warning: Attempt to read property "post_content" on null in /some-path/wp-content/themes/boldr-lite/functions.php on line 267

PHP Warning: Attempt to read property "post_content" on null in /some-path/wp-content/themes/boldr-lite/functions.php on line 268

I checked the functions.php and line number 267 and 268 as given below:

preg_match( '/<!--more(.*?)?-->/', $post->post_content )

|| preg_match( '/<!--nextpage-->/', $post->post_content )

And the full function including the above two lines as given below:

/*
 * Rewrite and replace wp_trim_excerpt() so it adds a relevant read more link
 * when the <!--more--> or <!--nextpage--> quicktags are used
 * This new function preserves every features and filters from the original wp_trim_excerpt
 */
function boldr_trim_excerpt( $text = '' ) {
	global $post;
	$raw_excerpt = $text;
	if ( '' === $text ) {
		$text = get_the_content( '' );
		$text = strip_shortcodes( $text );
		$text = apply_filters( 'the_content', $text );
		$text = str_replace( ']]>', ']]&gt;', $text );
		$excerpt_length = apply_filters( 'excerpt_length', 55 );
		$excerpt_more = apply_filters( 'excerpt_more', ' [...]' );
		$text = wp_trim_words( $text, $excerpt_length, $excerpt_more );

		/* If the post_content contains a <!--more--> OR a <!--nextpage--> quicktag
		 * AND the more link has not been added already
		 * then we add it now
		 */
		if (
			(
				preg_match( '/<!--more(.*?)?-->/', $post->post_content )
				|| preg_match( '/<!--nextpage-->/', $post->post_content )
			)
			&& strpos( $text, $excerpt_more ) === false
		) :
			$text .= $excerpt_more;
		endif;

	}
	return apply_filters( 'boldr_trim_excerpt', $text, $raw_excerpt );
}
remove_filter( 'get_the_excerpt', 'wp_trim_excerpt' );
add_filter( 'get_the_excerpt', 'boldr_trim_excerpt' );

Please help me to solve this issue, suggest me a corrected code