I have recently updated Unity from 2019.4.31 to 2021.2.x (any of the 2021.2.x has the same issue).
Since then I have had constant crashes that seem to be caused by ES3 in the latest versions of Unity, before this everything was working fine.
I can't save or play the project without it crashing. Even deleting the ES3 asset from the project still causes it to crash; when I click anything, Unity will start loading and I get a ES3Postprocessor.OnWillSaveAssets, even when ES3 is not in the project.
When I delete the asset from the project and reimport it, I get a Script Updating Consent popup about the ES3Prefab.cs. If I click yes, the project has no errors but trying to save or play causes crashes. If I press no, I get this error:
Here is some screenshots of the errors https://imgur.com/a/tqMoIuAAssets\Plugins\Easy Save 3\Scripts\ES3Prefab.cs(93,55): error CS0234: The type or namespace name 'PrefabStageUtility' does not exist in the namespace 'UnityEditor.Experimental.SceneManagement' (are you missing an assembly reference?)
I am not really sure what to do at this stage but currently the entire project is unusable. On the plus side, I use Unity Collab so I can go back to the version that was running on 2019.4.31, but I do need to update to the latest version of Unity. So I don't know if there is a better way to upgrade the project or maybe something happened when the project when it was opened in the newer editor.
Edit: Forgot to mention, I have already upgraded a project before this one and had no issues (From 2019.4.31 to 2021.2.8), it is just this project that is having these issues.