I think the issue causing that error message is actually with line 22 of the exiftool_keywords plugin main.inc.php file which invokes the shell_exec function to run exiftool.

Rather than returning a JSON string result from exiftool, shell_exec() is returning null there which breaks the json_decode() on line 23 producing the error. As far as I understand, a null result means that particular shell_exec() function is either producing an error or blank output.

I think it is possible that PHP on your system is unable to locate the external exiftool command on the specific path being used by that shell_exec() function. I do not have a macOS system to test with so if that is the case I don't know what the solution but it may mean the plugin needs some kind of environment variable set as part of the shell_exec call.

I am running ver 16.2.0 and get the same error.
I did a bit of Googling and found the following, suggesting this plugin is not compatible with my version of PHP (PHP: 8.3.6)


It looks like a simple fix for the plugin, but it's beyond my abilities so can't help:

Check the Code:
Locate the line in the specified file (main.inc.php on line 23) and ensure that the variable passed to json_decode() is not null.

Add a Condition:
Before calling json_decode(), add a check to ensure the variable is a valid JSON string. For example:
php

Insert this code:
if ($jsonString !== null) {
    $data = json_decode($jsonString);
}