NullReferenceException errors are thrown when any other window is docked in the position of the Package Manager window.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Dock with any other window.
3. Drag the other window > Dock in the position of Package Manager.

Actual Result:
NullReferenceException errors are thrown in the console.

Expected Result:
No errors in the console.

Reproducible in: 2019.3.0b11, 2019.2.13f1, 2019.1.0f1, 2018.4.11f1, 2018.3.0b6.

Working Fine in:
2020.1.0a13, 2018.3.0b5.

Environment:
Windows 10 & Mac 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PackageManager.UI.PackageList.OnEnterPanel (UnityEngine.UIElements.AttachToPanelEvent e) (at <b43e6d4802d64ea8bbdaa0bf64614d3b>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.UIElements.EventBase evt) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEventAtTargetPhase (UnityEngine.UIElements.EventBase evt) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.DefaultDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <b256487a5e4140809d026a943f161bc2>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <59837000ebf54955a9cb1d31caa86097>:0)