Can't get selected_text working

Describe the bug
I am trying to use the feature "save selected text to variable selected_text", with a AppleScript program (Run AppleScript (blocking). When i'm using the variable selected_text, it tells me that it is not defined. The fonction with selected_text works perfectly with the option search with Google, and i'd like to use this variable to do something else (notification with selected text in any app)

Device information:

  • Type of Mac: Macbook pro 15 late 2016
  • macOS version: 10.13
  • BetterTouchTool version: 2.530 (865)

(you can see I'm selecting something in Safari)