The plugin stop to work since the old "IDEA 11.1 EAP's" days (I also updates the plugin with every new version and nothing changes).
The plugin marks everything inside a *.kt file as an error with this obscure and mysterios message
"AssertionError:null"
Thanks in advance
YouTrack issue http://youtrack.jetbrains.com/issue/KT-1771
Thanks for the issue (http://youtrack.jetbrains.com/issue/KT-1771) Looking into it.
I saw the same when I tried to open Kotlin project in IDEA with installed kotlin plugin. All stdlib files were red.
This only means that the type checker failed at some point.