[IMGUI] NullReferenceException is thrown on docking any window.

NullReferenceException: Object reference not set to an instance of an object exception is thrown in the console.

Steps to repro:
1. Create a new project.
2. Undock any window and dock it.

Actual Result:
NullReferenceException: Object reference not set to an instance of an object exception thrown in the console.

Expected Result: Window docks without exception in the console.

Reproducible on:
2019.3.0a2.

Working Fine in:
2019.3.0a1, 2019.2.0b1.

Environment:
Windows & Mac.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:326)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect, System.Boolean floatingWindow, System.Boolean isBottomTab) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:387)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:356)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:297)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:492)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:475)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUI (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:465)
UnityEngine.UIElements.IMGUIContainer.HandleEvent (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:446)
UnityEngine.UIElements.MouseCaptureDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/Events/MouseCaptureDispatchingStrategy.cs:93)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:280)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:156)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:197)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:289)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:107)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)