LWRP - VR - Unity crashes when loading additive scene

I have a VR boss scene - that sets up the camera rig etc and then i load the environment scene separately. we've recently changed the project to use the LWRP graphics system. if i load the additive scene from Start() functions it works fine. If i wait 5 seconds and then load it it crashes Unity. The Stack trace points to LWRP camera setup functions. It actually works fine in builds it's just killing the editor which makes working on it a real pain. Anybody able to shed any light on this or make any suggestions? The reason it's a problem is that we can't load the additive scene from Start all the time. We have some other UI to go through first. Thanks.!