Enable nullability in TabPageCollectionEditor

src/System.Windows.Forms.Design/src/Resources/SR.resx

@@ -1623,5 +1623,7 @@ Press Ctrl+Enter to accept Text.</value>
   <data name="DesignBindingPickerHelpNodeProjectGroup" xml:space="preserve">
     <value>Contains project data sources belonging to this namespace.</value>
   </data>
-
+  <data name="CollectionEditorCreateInstanceError" xml:space="preserve">
+    <value>Neither {0} nor {1} can create an instance of {2}.</value>
+  </data>
 </root>

src/System.Windows.Forms.Design/src/System/ComponentModel/Design/CollectionEditor.cs

@@ -108,9 +108,22 @@ protected virtual object CreateInstance(Type itemType)
             }
         }
 
-        return itemType.UnderlyingSystemType == typeof(string)
-            ? string.Empty
-            : TypeDescriptor.CreateInstance(host, itemType, argTypes: null, args: null)!;
+        if (itemType.UnderlyingSystemType == typeof(string))
+        {
+            return string.Empty;
+        }
+
+        if (TypeDescriptor.CreateInstance(host, itemType, argTypes: null, args: null) is { } obj)
+        {
+            return obj;
+        }
+
+        throw new InvalidOperationException(
+            string.Format(
+            SR.CollectionEditorCreateInstanceError,
+            nameof(IDesignerHost),
+            nameof(TypeDescriptor),
+            itemType.FullName));
     }
 
     /// <summary>

src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/TabPageCollectionEditor.cs

@@ -1,8 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#nullable disable
-
 using System.ComponentModel.Design;
 
 namespace System.Windows.Forms.Design;
@@ -13,29 +11,31 @@ namespace System.Windows.Forms.Design;
 /// </summary>
 internal class TabPageCollectionEditor : CollectionEditor
 {
-    public TabPageCollectionEditor() : base(typeof(TabControl.TabPageCollection))
+    public TabPageCollectionEditor()
+        : base(typeof(TabControl.TabPageCollection))
     {
     }
 
     /// <summary>
     ///  Sets the specified collection to have the specified array of items.
     /// </summary>
-    protected override object SetItems(object editValue, object[] value)
+    protected override object? SetItems(object? editValue, object[]? value)
     {
-        var tabControl = Context.Instance as TabControl;
+        var tabControl = Context?.Instance as TabControl;
         tabControl?.SuspendLayout();
 
-        object retValue = base.SetItems(editValue, value);
+        object? retValue = base.SetItems(editValue, value);
 
         tabControl?.ResumeLayout();
+
         return retValue;
     }
 
     protected override object CreateInstance(Type itemType)
     {
         object instance = base.CreateInstance(itemType);
 
-        TabPage tabPage = instance as TabPage;
+        TabPage tabPage = (TabPage)instance;
         tabPage.UseVisualStyleBackColor = true;
 
         return tabPage;