I'm unable to reproduce that error. It works fine here with KM 8.0.1 on macOS 10.13.
I don't know what "Invalid File" exactly means. Maybe try to reinstall the plugin?
After reinstalling action plugins — if the action already is inserted in a macro — you should reload KM and KM engine:
tell application "Keyboard Maestro" to reload
tell application "Keyboard Maestro Engine" to reload
or simply relaunch them.
Edit:
The issue has been resolved; see posts below.
Update for KM7 (26 Sep 2017, 21:20 Z) 
Manipulate a Display Text Window (KM7).zip (7.7 KB)
…and…
Update for KM8 (26 Sep 2017, 21:22 Z) 
Manipulate a Display Text Window (KM8).zip (7.5 KB)
I have removed the "Focus OK button" checkbox, since this has become the default with KM8. So, download this version only if you are using KM8+.