Failed to destroy error thrown on clicking Continue button in Popup appears after minimizing Package Manager

Steps to Reproduce:

1. Create a new Project
2. Open Window > Package Manager
3. Dock Package Manager window
4. Select Package Manager window and click Shift+Space to maximize
5. Select Package Manager window and click Shift+Space to minimize
6. Click on Continue button

Expected Result:
Popup closes without any error

Actual Result:
Failed to destroy editor windows: #1 error thrown

Reproducible on: 2020.1.0a15

Working fine on: 2020.1.0a14, 2019.3.0f1

Environment: Windows 10 and Mac 10.14

Stack Trace:

1. Maximization failed: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
at UnityEditor.PackageManager.UI.PackageManagerWindow.get_packageList () [0x00001] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
at UnityEditor.PackageManager.UI.PackageManagerWindow.OnLostFocus () [0x00001] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
at (wrapper managed-to-native) System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod,object,object[],System.Exception&)
at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in <437ba245d8404784b9fbab9b439ac908>:0
--- End of inner exception stack trace ---
at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00048] in <437ba245d8404784b9fbab9b439ac908>:0
at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <437ba245d8404784b9fbab9b439ac908>:0
at UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) [0x0000f] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
at UnityEditor.HostView.DeregisterSelectedPane (System.Boolean clearActualView, System.Boolean sendEvents) [0x00143] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
at UnityEditor.DockArea.AddTab (System.Int32 idx, UnityEditor.EditorWindow pane, System.Boolean sendPaneEvents) [0x00001] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
at UnityEditor.WindowLayout.Unmaximize (UnityEditor.EditorWindow win) [0x00155] in <1df4bfcafa864ca99de2ab91a95ebbc4>:0
UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32)

2. Failed to destroy editor windows: #1
PackageManagerWindow
UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32)