Github: https://github.com/Unity-Technologies/InputSystem/issues/681

Repro:
1. Download package and project at: https://github.com/Unity-Technologies/InputSystem
2. Open Preferences window (Edit -> Preferences...)
3. With the Preferences window open, enter then exit Playmode
4. Observe Console window

Actual: When Preferences window is open, exiting Playmode with ISX generates exception.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.SettingsProvider.Repaint () (at C:/buildslave/unity/build/Editor/Mono/Settings/SettingsProvider.cs:123)
UnityEngine.InputSystem.Editor.InputSettingsProvider.OnSettingsChange () (at Packages/com.unity.inputsystem/InputSystem/Editor/Settings/InputSettingsProvider.cs:307)
UnityEngine.InputSystem.InputManager.ApplySettings () (at Packages/com.unity.inputsystem/InputSystem/InputManager.cs:2178)
UnityEngine.InputSystem.InputSettings.OnChange () (at Packages/com.unity.inputsystem/InputSystem/InputSettings.cs:301)
UnityEngine.InputSystem.InputSystem.OnPlayModeChange (UnityEditor.PlayModeStateChange change) (at Packages/com.unity.inputsystem/InputSystem/InputSystem.cs:1671)
UnityEngine.InputSystem.LowLevel.NativeInputRuntime.OnPlayModeStateChanged (UnityEditor.PlayModeStateChange value) (at Packages/com.unity.inputsystem/InputSystem/NativeInputRuntime.cs:223)
UnityEditor.EditorApplication.Internal_PlayModeStateChanged (UnityEditor.PlayModeStateChange state) (at C:/buildslave/unity/build/Editor/Mono/EditorApplication.cs:302)