Mixedrealitytoolkit-unity: "Add to Scene and Configure" doesn't gracefully fail when no camera is present in the scene.

[Cameron-Micka]

Describe the bug

When the "Mixed Reality Toolkit->"Add to Scene and Configure..." menu item is selected in a scene without a camera a handful of null reference exceptions occur and the MRTK is not configured correctly.

The project we are seeing this occur in loads a camera dynamically at runtime based on certain project restrictions.

To reproduce

Select "Mixed Reality Toolkit->"Add to Scene and Configure..." in a scene without a camera.

Expected behavior

All exceptions are gracefully handled. Optionally, anything relying on the Camera or Camera Cache could automatically setup a main camera for the user.

[Railboy]

Related: #3890

[chrisfromwork]

Assigning to @Yoyozilla to find an owner.

[Yoyozilla]

Cameron, could you please check that your issue is fixed by Lar's PR linked above?

[Cameron-Micka]

Thanks! I'll give this a try when I get a chance.