In my case, this was caused by having a VR project that I was running on a new version of Unity (2018.3.0f2) without having the OpenVR package installed.
Easy fix: Open the Package Manager (Window->Package Manager), select “OpenVR” in the package list and click install. Restart Unity
Grumpy.Code.Dude. 
THANK YOU, got this problem from the last update and it was ruining everything.
That saved my day TAT thanks!!!