When a new Tile Palette is created with Tile Palette window docked in the Editor and then the project is re-imported then ArgumentNullException appears in the console, Refer attached video.

Steps to repro:
1. Create a new Project with 2D Template
2. Window > 2D > Tile Palette > Docked
3. Create a New Tile Palette
4. Reimport the Project

Actual Result:
ArgumentNullException is thrown on Reimporting the Project if the Tile Palette window docked and New Palette created.

Expected Result:
Projet is reimported without any exception even if the Tile Palette window was docked and New Palette was created

Reproducible in:
2020.2.0b2 with Package version 1.0.0

Working fine in
2020.2.0b1, 2020.1.4f1 with Package version 1.0.0

Environment:
Occurring on Windows 10 & macOS 10.15.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: collection
System.Collections.Generic.HashSet`1[T]..ctor (System.Collections.Generic.IEnumerable`1[T] collection, System.Collections.Generic.IEqualityComparer`1[T] comparer) (at <351e49e2a5bf4fd6beabb458ce2255f3>:0)
System.Collections.Generic.HashSet`1[T]..ctor (System.Collections.Generic.IEnumerable`1[T] collection) (at <351e49e2a5bf4fd6beabb458ce2255f3>:0)
UnityEditor.PropertyEditor.OnPrefabInstanceUnpacked (UnityEngine.GameObject unpackedPrefabInstance) (at <3d7c86b365634083bceadac5c7f36490>:0)
UnityEditor.PrefabUtility.UnpackPrefabInstance (UnityEngine.GameObject instanceRoot, UnityEditor.PrefabUnpackMode unpackMode, UnityEditor.InteractionMode action) (at <3d7c86b365634083bceadac5c7f36490>:0)
UnityEditor.Tilemaps.GridPaintPaletteWindow.ResetPreviewInstance () (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteWindow.cs:556)
UnityEditor.Tilemaps.GridPaintPaletteWindow.InitPreviewUtility () (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteWindow.cs:618)
UnityEditor.Tilemaps.GridPaintPaletteWindow.get_previewUtility () (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteWindow.cs:423)
UnityEditor.Tilemaps.GridPaintPaletteClipboard.get_previewUtility () (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteClipboard.cs:70)
UnityEditor.Tilemaps.GridPaintPaletteClipboard.OnViewSizeChanged (UnityEngine.Rect oldSize, UnityEngine.Rect newSize) (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteClipboard.cs:403)
UnityEditor.Tilemaps.GridPaintPaletteClipboard.set_guiRect (UnityEngine.Rect value) (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteClipboard.cs:46)
UnityEditor.Tilemaps.GridPaintPaletteWindow.OnClipboardGUI (UnityEngine.Rect position) (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteWindow.cs:1213)
UnityEditor.Tilemaps.GridPaintPaletteWindow.OnGUI () (at F:/build1/2020.2.0b2/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.2d.tilemap/Editor/GridPaintPaletteWindow.cs:486)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at <3d7c86b365634083bceadac5c7f36490>:0)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect) (at <3d7c86b365634083bceadac5c7f36490>:0)
UnityEditor.DockArea.OldOnGUI () (at <3d7c86b365634083bceadac5c7f36490>:0)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.IMGUIContainer.DoIMGUIRepaint () (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIR.RenderChainCommand.ExecuteNonDrawMesh (UnityEngine.UIElements.UIR.DrawParams drawParams, System.Single pixelsPerPoint, System.Exception& immediateException) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
Rethrow as ImmediateModeException
UnityEngine.UIElements.UIR.RenderChain.Render () (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIRRepaintUpdater.Update () (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.Panel.UpdateForRepaint () (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <25b3b10f7ceb493e97ab16f983c3ca81>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at