[Editor] Null Reference exception is thrown at the time of switching Aspect Ratio in GameView

1. Open a new Project
2. Go to Game window
3. Change the aspect ratio

Expected Result: Aspect ratio should be changed gracefully
Actual Result: NullReferenceException is thrown

Environment
Windows 10 and Mac 10.13

Reproducible with
2019.1.0a9, 2018.3.0b11

Working Fine
2019.1.0a8, 2018.3.0b10

Note: This issue is also occurring in the scenario given below:

1. Hierarchy -> 2D - Tilemap(Create 2-3 tile maps)
2. Window -> Tile Palette
3. Tile Palette -> Create new Palette(Create 2-3 tile palettes.
4. Switch between the tile palettes and tilemaps.

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PopupWindow.FitWindowToContent () (at C:/buildslave/unity/build/Editor/Mono/GUI/PopupWindow.cs:97)
UnityEditor.PopupWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/PopupWindow.cs:92)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <ac823e2bb42b41bda67924a45a0173c3>: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 <ac823e2bb42b41bda67924a45a0173c3>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <ac823e2bb42b41bda67924a45a0173c3>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:365)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:359)
UnityEditor.HostView.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:152)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:271)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:443)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:426)
UnityEngine.UIElements.IMGUIContainer.HandleEvent (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:406)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/Events/IEventDispatchingStrategy.cs:71)
UnityEngine.UIElements.MouseEventDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/Events/MouseEventDispatchingStrategy.cs:58)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:275)
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:151)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:154)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:250)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:68)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)