Packman window appears blank on restarting the Project after clicking on quit on Revert All Window Layouts popup

Steps to Reproduce:

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 the quit button
7. Restart the Project

Expected Result:
Package Manager window appears correctly

Actual Result:
- Package Manager window appears blank
- Nullreference Exception thrown if click on PackMan window

Reproducible on: 2020.1.0a15

Working fine on: 2020.1.0a14, 2019.3.0f1

Environment: Windows 10 and Mac 10.14

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PackageManager.UI.PackageManagerWindow.get_packageList () (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEditor.PackageManager.UI.PackageManagerWindow.OnLostFocus () (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <437ba245d8404784b9fbab9b439ac908>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEditor.HostView.Invoke (System.String methodName) (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEditor.HostView.OnLostFocus () (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)