Undefinied variable

Hello,

Today I have been doing little maintance of site and have deactivated all plugins.

Then I enabled debug mode and started activating one by one to see if any causes error.

When I activated WP Slimstat some error showed.

Here is image: http://s2.postimg.org/s31xh1p89/slimstat.jpg

What that means and how to fix it?

Any sugesstions?

https://ww.wp.xz.cn/plugins/wp-slimstat/