Hoping this doesn't muddy the waters, but that it helps in some way. I've found that this happens quite a bit while switching between visual studio, editing scripts, and going back to Unity. If you have Unity in play mode while saving a script, it does this.

Actually, around 80% of the time, Unity completely crashes under these circumstances. If lucky, I just get this error.

Workaround is to NOT save scripts while Unity is in run mode. Needless to say, it would be great if there were a better workaround.

Thanks for your feedback.

Yes, we are aware of the "editing scripts while in play mode" problem. We are working to solve it. In theory now, you should get a message saying that scripts cannot be edited while in play mode (this is a temporal workaround). But even sometimes this message is not properly shown. I suppose this is your case.