This has now been fixed, but the update will be delayed about 3 weeks I'm afraid
To be clear, this happens when pressing the "Open shader in Shader Forger" button in the inspector.
SF doesn't support beta versions I'm afraid. The next update will be for 5.6, and after that it'll probably start to target Unity 2017.
Got the same issue and found that it boils down to the way the reference to the Camera object in the SF_PreviewWindow. m_camera. SF tries to read the m_camera variable from the PreviewRenderUtility class, which has been made obsolete and Unity is suggesting to use the 'camera' getter instead. The same goes for the m_Light variable.
By assigning pruCam / pruLights like this in seems to solve the issue:
// class: SF_PreviewWindow, function: SetupPreview
PreviewRenderUtility pru = pruRef as PreviewRenderUtility;
pruCam = pru.camera;
pruLights = pru.lights;
I've tested this for Unity 2017 (beta 5) and seems to work at first glance. Can't say if anything else is broken due to obsolete fields in 2017 or other potential incompatibilities.
Customer support service by UserEcho