Steps to reproduce:
1. Open new project
2. Create a script that opens an editor window (Or just use the example script from https://docs.unity3d.com/ScriptReference/EditorWindow.html)
3. Open the window
4. Copy the script to another location in your disk
5. Delete the original
6. Observe the window saying it "Failed to Load"
7. Add the script again to your project
8. The window loses it's docking tab and you can not close it anymore (Getting an error in the console every time)

Short version of the Error: (full version attached to case)
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.ThrowHelper.ThrowArgumentOutOfRangeException (System.ExceptionArgument argument, System.ExceptionResource resource) (at <437ba245d8404784b9fbab9b439ac908>:0)

Use case: Having a window created by a package open when updating to a different package version (Remove and then Add via package manager)

Reproduced on: 2019.1.14f1, 2019.2.13f1, 2019.3.0b12, 2020.1.0a14