Issue seems to be stemming from a third party asset's (tk2d) compatibility issues with U6. The crash originates from the tk2dCameraEditor.cs file, specifically PreviewWindowFunc's previewCamera.Render() call.
The current recommended practice would be to get a temporary render texture using RenderTexture.GetTemporary, set it as the target of the preview camera, render the preview camera and finally use Graphics.DrawTexture to output the render texture to the preview window. We've tested this approach within PreviewWindowFunc and it does not crash the editor.
For an example on how to do camera previews in Scene View, please take a look at: https://github.com/Unity-Technologies/UnityCsReference/blob/master/Modules/SceneView/CameraOverlay.cs