GitHub: https://github.com/Unity-Technologies/InputSystem/issues/436
Forum: https://forum.unity.com/threads/errors-with-new-input-system-when-exiting-play-mode-solved.632644/

Repro:
1. Download package and project at: https://github.com/Unity-Technologies/InputSystem
2. Open SimpleDemo scene at: Assets -> Demo -> SimpleDemo
3. Select "Player" Gameobject and open the attached script "SimpleController_UsingActions_InAsset"
4. In the C# script file, find "OnDisable" function and remove "controls.Disable();" from it.
5. Save the script, go back to Unity Editor and Enter Play Mode
6. Exit Play Mode and observe the issue

Actual: If user doesn't disabled Input Action manually when exiting Play Mode in Editor, error and exception keep popping up in Console window until restart the Editor.
Expected: No unmanaged memory leak. If users are required to disable Input Action manually, the error should be more informative and relevant.

From GitHub, Rene:
Automatic cleanup of actions (i.e. disabling them and releasing all their unmanaged memory) does not seem to work reliably and lead to errors about unmanaged memory being leaked.