Strange bug- all items in hierarchy have become invisible

Have encountered a strange bug with a current project. Running Unity 5.4.1f1
I can select items in my hierarchy but they are invisible(see attached)

Not sure if its related but also get a compile error with VivenavMeshEditor.cs

cs(149,66): error CS0234: The type or namespace name `AI' does not exist in the namespace `UnityEngine'. Are you missing an assembly reference?

Also might be related, I recently installed a 2nd copy of Unity beta 5.5 to run the new experimental Image Sequencer.

Is it possible opening this project with the new Unity 5.5 beta might have messed things up?

 

The bad news. You cannot downgrade from 5.5 to 5.4, ever, if you've modified your scenes, prefabs or particles.

They created an (IMHO unnecessary) new serializable format in 5.5 that 5.4 cannot understand to remove a handful of redundant data points for some serializable data types.

If you are using binary format, Unity 5.4 will appropriately tell you that it cannot open the file

If you are using a text format, there are bugs still present in 5.4 and 5.5 that does not warn you. And when it sees an invalid file format, it disables the asset. And when it tries to access the redundant property that they removed, it can't find the reference, and so it creates a blank object.

To repeat, you cannot downgrade from 5.5 to 5.4 with most prefabs, scenes and particles. I did my best to learn the format and roll my own tool to downgrade, but at the end of the day the only people who know what changed (and why) and who can create a proper downgrade tool is the Unity team themselves.