If you're interested in the technical details:
The
GuiOpenEvent#gui
field was previously public, but it was changed to a private field with a public getter method in Forge 1.9-12.16.0.1805-1.9. Since the field is private, it can no longer be accessed directly; the getter method must be used instead.
I dunno what that means
Also, this same error happens for any mod I try to install, so I dunno what's goin on