How to debug "Problems occurs on injecting env vars as a build wrap: null"

I use Groovy to inject variables into the build process using "EnvInject Plugin" plugin:

My script is saved under "Evaluated Groovy script" text area.

If the script is working, everything is fine. But if some error occurs, I always get the same error message:

[EnvInject] - [ERROR] - [EnvInject] - [ERROR] - Problems occurs on injecting env vars as a build wrap: null

Is there any way to get the actual exception or even exception message?