Enable nullability in some Control members (#7309)

## Proposed changes

- Enable nullability in some Control members.

src/System.Windows.Forms/src/System/Windows/Forms/ButtonBase.cs

@@ -4,6 +4,7 @@
 
 using System.ComponentModel;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Design;
 using System.Windows.Forms.ButtonInternal;
@@ -645,6 +646,7 @@ internal bool ShowToolTip
 
  [Editor("System.ComponentModel.Design.MultilineStringEditor, " + AssemblyRef.SystemDesign, typeof(UITypeEditor)),
  SettingsBindable(true)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/src/System/Windows/Forms/Form.cs

@@ -2960,7 +2960,7 @@ private void ApplyClientSize()
  /// Assigns a new parent control. Sends out the appropriate property change
  /// notifications for properties that are affected by the change of parent.
  /// </summary>
- internal override void AssignParent(Control value)
+ internal override void AssignParent(Control? value)
  {
  // If we are being unparented from the MDI client control, remove
  // formMDIParent as well.

src/System.Windows.Forms/src/System/Windows/Forms/GroupBox.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Text;
 using System.Windows.Forms.Layout;
@@ -240,6 +241,7 @@ public FlatStyle FlatStyle
  }
 
  [Localizable(true)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/src/System/Windows/Forms/ListBox.cs

@@ -1088,19 +1088,20 @@ public bool Sorted
  [EditorBrowsable(EditorBrowsableState.Advanced)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  [Bindable(false)]
- public override string? Text
+ [AllowNull]
+ public override string Text
  {
  get
  {
  if (SelectionMode != SelectionMode.None && SelectedItem is not null)
  {
  if (FormattingEnabled)
  {
- return GetItemText(SelectedItem);
+ return GetItemText(SelectedItem) ?? string.Empty;
  }
  else
  {
- return FilterItemOnProperty(SelectedItem)?.ToString();
+ return FilterItemOnProperty(SelectedItem)?.ToString() ?? string.Empty;
  }
  }
  else

src/System.Windows.Forms/src/System/Windows/Forms/ListView.cs

@@ -6,6 +6,7 @@
 using System.ComponentModel;
 using System.ComponentModel.Design;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Design;
 using System.Globalization;
@@ -1649,6 +1650,7 @@ public ImageList? StateImageList
  [Browsable(false)]
  [EditorBrowsable(EditorBrowsableState.Never)]
  [Bindable(false)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;
@@ -6131,7 +6133,7 @@ private unsafe bool WmNotify(ref Message m)
  _columnHeaderClicked = null;
  _columnHeaderClickedWidth = -1;
 
- ISite site = Site;
+ ISite? site = Site;
 
  if (site is not null)
  {

src/System.Windows.Forms/src/System/Windows/Forms/Printing/PrintPreviewControl.cs

@@ -4,6 +4,7 @@
 
 using System.ComponentModel;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Printing;
 using static Interop;
@@ -208,6 +209,7 @@ public override RightToLeft RightToLeft
  [EditorBrowsable(EditorBrowsableState.Never)]
  [Bindable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/src/System/Windows/Forms/ProgressBar.cs

@@ -435,6 +435,7 @@ public int Step
  [Browsable(false)]
  [EditorBrowsable(EditorBrowsableState.Never)]
  [Bindable(false)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/src/System/Windows/Forms/ScrollBar.cs

@@ -327,6 +327,7 @@ public int SmallChange
  [EditorBrowsable(EditorBrowsableState.Never)]
  [Bindable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/src/System/Windows/Forms/TabPage.cs

@@ -481,9 +481,9 @@ public string ToolTipText
  /// Assigns a new parent control. Sends out the appropriate property change notifications for
  /// properties that are affected by the change of parent.
  /// </summary>
- internal override void AssignParent(Control value)
+ internal override void AssignParent(Control? value)
  {
- if (value is not null && !(value is TabControl))
+ if (value is not null && value is not TabControl)
  {
  throw new ArgumentException(string.Format(SR.TabControlTabPageNotOnTabControl, value.GetType().FullName));
  }

src/System.Windows.Forms/src/System/Windows/Forms/TrackBar.cs

@@ -492,6 +492,7 @@ public int SmallChange
  [Browsable(false)]
  [EditorBrowsable(EditorBrowsableState.Never)]
  [Bindable(false)]
+ [AllowNull]
  public override string Text
  {
  get => base.Text;

src/System.Windows.Forms/tests/IntegrationTests/UIIntegrationTests/DesignBehaviorsTests.cs

@@ -331,7 +331,7 @@ protected override void OnPaintAdornments(PaintEventArgs pe)
  {
  base.OnPaintAdornments(pe);
 
- pe.Graphics.DrawString($"Design time \n{Control.Site.Name} !", Control.Font, SystemBrushes.WindowText, new PointF(12, 12));
+ pe.Graphics.DrawString($"Design time \n{Control.Site?.Name} !", Control.Font, SystemBrushes.WindowText, new PointF(12, 12));
  }
  }
  }