NullReferenceException has thrown after maximizing and minimizing the ProBuilder window, once this is done it won't respond.

Steps to repro:
1. Create a new project.
2. Install "ProBuilder" package from Window > Package Manager.
3. Tools > ProBuilder > ProBuilder Window
4. Dock 'ProBuilder' window.
5. Check "Maximize" from window options drop-down or (Shift+space)
6. Uncheck "Maximize" from window options drop-down or (Shift+space)

Actual Result
Null reference exception is thrown in the console and ProBuilder window won't respond.

Expected Result:
ProBuilder window should be maximized and minimized gracefully

Environment:
Windows and Mac

Occurring with:
2019.1.0a6, 2018.3.0b7, 2018.3.0b1, 2018.2.0f2, 2018.1.0f2

Probuilder package version: 3.0.9

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
ProBuilder.Actions.SelectHole.IsHidden () (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/MenuActions/Selection/SelectHole.cs:44)
ProBuilder.EditorCore.pb_EditorToolbar.IsActionValid (ProBuilder.EditorCore.pb_MenuAction action) (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_EditorToolbar.cs:219)
ProBuilder.EditorCore.pb_EditorToolbar.OnGUI () (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_EditorToolbar.cs:232)
ProBuilder.EditorCore.pb_Editor.OnGUI () (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_Editor.cs:377)
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:342)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:336)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:310)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect, System.Boolean customBorder, System.Boolean floatingWindow, System.Boolean isBottomTab) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:363)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:322)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:244)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)