Valve's SteamVR Input Plugin

Hey, so I was implementing the plugin all day and it was going well. Then I closed and opened the editor and now I get all these errors:

The “SteamVR Skeletal Anim Pose” and “SteamVR Set Wrist Transform” nodes have disappeared from my Anim graph. I put them back, then the next time I restart the editor they’ve gone again and the errors are back!!