Undefined property: Grayscale_Image_Editor

It definitely is an error due to the plugin, because it has to do with Grayscale_Image_Editor class, which is defined in the plugin.

Could you tell me which version of WordPress you are using ?
The plugin will not work just by activating it. You have to insert a php code in your theme file to tell the plugin to generate grayscale image for specific image size. Could you provide the code you are using ?

Hi, now I’m using WordPress 3.8, but when I use WordPress 3.7.1 it back to me the same error

in the function.php I have this code
grayscale_add_image_size('thumb_actualidad', 500, 500, true, true);

and for print this images on theme I’m use this:
<?php the_post_thumbnail( 'thumb_actualidad-gray' ); ?>

thanks!

OK, I was able to reproduce the issue with WP 3.8. The Warning message is displayed when the picture is uploaded, but it does not prevent the plugin to work.
I will try to correct this as soon as possible.

Except this message, does it work as you expect ?

No, I can’t upload any image, I try to upload some image as featured image and display an error and I can’t continue with the process

thanks again

the problem is solve, I made a clear installation of wordpress, I import the database, upload plugin, theme and upload folders. and the plugin works again.. I don’t know how but is works again.

thanks.. you can visit the site ??

I guess that re-installing Worpress set the WP_DEBUG option to false, thus hiding the error message and restoring the grayscale functionality.
To test this, you can edit the wp-config.php file and change the following line :
define(‘WP_DEBUG’, false);
to :
define(‘WP_DEBUG’, true);
and check that the error message reappeared.
Of course, it is recommended to set WP_DEBUG to false on a production website. See Debugging_in_WordPress in the codex for more information.

I will investigate deeper to understand the cause of the message and update the plugin as soon as I find a solution.