Shop Isle Error Function Not Found on Update

ISSUE:

I’m editing posts on my local website when from nowhere I get this error from the included XDebug output:

> call_user_func_array() expects parameter 1 to be a valid callback, function ‘shop_isle_custom_add_save’ not found or invalid function name in [class-wp-hook.php]

However, I can’t find any specific thing that I’m doing besides editing or updating posts prior to receiving this error. Also, if I refresh the page, the error goes away until the next save. Everything is updated already (WP version, theme, and all plugins) and I haven’t created any new functions.

MY QUESTIONS:

What is causing this error and how do I resolve and prevent it from happening further?

MY SYSTEM:

• WordPress 4.8.2
• Windows 10 64/Wamp
• Server: localhost

Any help is appreciated.

FULL ERROR MESSAGE FROM XDEBUG:

From “post.php“: (upon attempting to update a post)

( ! ) Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'shop_isle_custom_add_save' not found or invalid function name in C:\wamp64\www\...\wp-includes\class-wp-hook.php on line 300
Call Stack
#	Time	Memory	Function	Location
1	0.0016	363168	{main}( )	...\post.php:0
2	0.6120	30505952	edit_post( )	...\post.php:193
3	0.6332	30558816	wp_update_post( )	...\post.php:378
4	0.6395	30605688	wp_insert_post( )	...\post.php:3583
5	0.7143	30696952	do_action( )	...\post.php:3484
6	0.7143	30697720	WP_Hook->do_action( )	...\plugin.php:453
7	0.7143	30697800	WP_Hook->apply_filters( )	...\class-wp-hook.php:323
8	0.7144	30699016	call_user_func_array:{C:\wamp64\www\...\wp-includes\class-wp-hook.php:300} ( )	...\class-wp-hook.php:300
9	0.7144	30699320	wp_save_post_revision( )	...\class-wp-hook.php:300
10	0.7248	30725872	_wp_put_post_revision( )	...\revision.php:186
11	0.7249	30728288	wp_insert_post( )	...\revision.php:300
12	0.7391	30752624	do_action( )	...\post.php:3510
13	0.7392	30753904	WP_Hook->do_action( )	...\plugin.php:453
14	0.7392	30753984	WP_Hook->apply_filters( )	...\class-wp-hook.php:323
15	0.7392	30755152	call_user_func_array:{C:\wamp64\www\...\wp-includes\class-wp-hook.php:300} ( )	...\class-wp-hook.php:300

( ! ) Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'shop_isle_custom_add_save' not found or invalid function name in C:\wamp64\www\...\wp-includes\class-wp-hook.php on line 300
Call Stack
#	Time	Memory	Function	Location
1	0.0016	363168	{main}( )	...\post.php:0
2	0.6120	30505952	edit_post( )	...\post.php:193
3	0.6332	30558816	wp_update_post( )	...\post.php:378
4	0.6395	30605688	wp_insert_post( )	...\post.php:3583
5	0.8644	30725936	do_action( )	...\post.php:3510
6	0.8644	30726560	WP_Hook->do_action( )	...\plugin.php:453
7	0.8644	30726640	WP_Hook->apply_filters( )	...\class-wp-hook.php:323
8	0.8645	30727616	call_user_func_array:{C:\wamp64\www\...\wp-includes\class-wp-hook.php:300} ( )	...\class-wp-hook.php:300

( ! ) Warning: Cannot modify header information - headers already sent by (output started at C:\wamp64\www\...\wp-includes\class-wp-hook.php:300) in C:\wamp64\www\...\wp-admin\post.php on line 197
Call Stack
#	Time	Memory	Function	Location
1	0.0016	363168	{main}( )	...\post.php:0
2	0.9412	30665232	setcookie ( )	...\post.php:197

( ! ) Warning: Cannot modify header information - headers already sent by (output started at C:\wamp64\www\...\wp-includes\class-wp-hook.php:300) in C:\wamp64\www\...\wp-includes\pluggable.php on line 1210
Call Stack
#	Time	Memory	Function	Location
1	0.0016	363168	{main}( )	...\post.php:0
2	0.9719	30665216	redirect_post( )	...\post.php:200
3	0.9730	30665656	wp_redirect( )	...\post.php:1868
4	0.9732	30676792	header ( )	...\pluggable.php:1210