How to reproduce:
1. Create a new 3D project
2. Open Package Manager window (Window > Package Manager)
3. Install Input System package
4. In the dialog box, asking to enable backends, click "Yes"

Expected result: Editor restarts and Active Input Handling changes
Actual result: Editor crashes

Reproducible with: 2020.3.21f1

Some of stack trace:
0x00007ff79a47c552 (Unity) RepaintController::RenderPlayModeViewCameras
0x00007ff79ae3745e (Unity) EditorGUIUtility::RenderPlayModeViewCamerasInternal
0x00007ff79b85e8b9 (Unity) EditorGUIUtility_CUSTOM_RenderPlayModeViewCamerasInternal_Injected
0x0000019d417dc0e7 (Mono JIT Code) (wrapper managed-to-native) UnityEditor.EditorGUIUtility:RenderPlayModeViewCamerasInternal_Injected (UnityEngine.RenderTexture,int,UnityEngine.Vector2&,bool,bool)
0x0000019d417dc003 (Mono JIT Code) UnityEditor.EditorGUIUtility:RenderPlayModeViewCamerasInternal (UnityEngine.RenderTexture,int,UnityEngine.Vector2,bool,bool)
0x0000019d417da67b (Mono JIT Code) UnityEditor.PlayModeView:RenderView (UnityEngine.Vector2,bool)
0x0000019d417a6293 (Mono JIT Code) UnityEditor.GameView:OnGUI ()
0x0000019d417a2d63 (Mono JIT Code) UnityEditor.HostView:InvokeOnGUI (UnityEngine.Rect,UnityEngine.Rect)
0x0000019d417a2ad3 (Mono JIT Code) UnityEditor.DockArea:DrawView (UnityEngine.Rect,UnityEngine.Rect)
0x0000019d41796ac3 (Mono JIT Code) UnityEditor.DockArea:OldOnGUI ()
0x0000019d4150a040 (Mono JIT Code) UnityEngine.UIElements.IMGUIContainer:DoOnGUI (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect,bool,UnityEngine.Rect,System.Action,bool)
0x0000019d4150904b (Mono JIT Code) UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect,System.Action,bool)
0x0000019d41507773 (Mono JIT Code) UnityEngine.UIElements.IMGUIContainer:DoIMGUIRepaint ()
0x0000019d4150472b (Mono JIT Code) UnityEngine.UIElements.UIR.RenderChainCommand:ExecuteNonDrawMesh (UnityEngine.UIElements.UIR.DrawParams,single,System.Exception&)
0x0000019d415028eb (Mono JIT Code) UnityEngine.UIElements.UIR.UIRenderDevice:EvaluateChain (UnityEngine.UIElements.UIR.RenderChainCommand,UnityEngine.Material,UnityEngine.Material,UnityEngine.Texture,UnityEngine.Texture,UnityEngine.Texture,single,Unity.Collections.NativeSlice`1<UnityEngine.UIElements.UIR.Transform3x4>,Unity.Collections.NativeSlice`1<UnityEngine.Vector4>,UnityEngine.MaterialPropertyBlock,bool,System.Exception&)
0x0000019d414fdb93 (Mono JIT Code) UnityEngine.UIElements.UIR.RenderChain:Render ()
0x0000019d414e85d3 (Mono JIT Code) UnityEngine.UIElements.UIRRepaintUpdater:Update ()
0x0000019c496fab1e (Mono JIT Code) UnityEngine.UIElements.VisualTreeUpdater:UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase)
0x0000019c496faa4b (Mono JIT Code) UnityEngine.UIElements.Panel:UpdateForRepaint ()
0x0000019c496fa583 (Mono JIT Code) UnityEngine.UIElements.Panel:Repaint (UnityEngine.Event)
0x0000019c496f9de7 (Mono JIT Code) UnityEngine.UIElements.UIElementsUtility:DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel)
0x0000019c496f9b3b (Mono JIT Code) UnityEngine.UIElements.UIElementsUtility:UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (int,intptr,bool&)
0x0000019c496f993f (Mono JIT Code) UnityEngine.UIElements.UIEventRegistration:ProcessEvent (int,intptr)
0x0000019c496f987b (Mono JIT Code) UnityEngine.UIElements.UIEventRegistration/<>c:<.cctor>b__1_2 (int,intptr)
0x0000019c496f96dd (Mono JIT Code) UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)
0x0000019c496f978e (Mono JIT Code) (wrapper runtime-invoke) <Module>:runtime_invoke_void_int_intptr_intptr& (object,intptr,intptr,intptr)
0x00007ff85267e660 (mono-2.0-bdwgc) [mini-runtime.c:2827] mono_jit_runtime_invoke