An object reference is required for the nonstatic field 'listBox1'"

Unit Unit1;
 
interface
 
uses System, System.Drawing, System.Windows.Forms;
type
  Form1 = class(Form)
    procedure button1_Click(sender: Object; e: EventArgs);
    procedure listBox1_SelectedIndexChanged(sender: Object; e: EventArgs);
    procedure button2_Click(sender: Object; e: EventArgs);
  {$region FormDesigner}
    {$resource Unit1.Form1.resources}
  internal
    label1: &Label;
    button1: Button;
    button2: Button;
    button3: Button;
    button4: Button;
    listBox1: ListBox;
    components: System.ComponentModel.IContainer;
    {$include Unit1.Form1.inc}
  {$endregion FormDesigner}
  public
    constructor;
    begin
      InitializeComponent;
    end;
  end;
 
implementation
uses Unit2;
procedure Form1.button1_Click(sender: Object; e: EventArgs);
begin
  (new Unit2.Form2).ShowDialog
end;
 
procedure Form1.listBox1_SelectedIndexChanged(sender: Object; e: EventArgs);
begin
  
end;
 
procedure Form1.button2_Click(sender: Object; e: EventArgs);
begin
  listBox1.Items.Add('Makar')
end;
 
end.

Unit Unit2;
 
interface
 
uses System, System.Drawing, System.Windows.Forms, Unit1;
 
type
  Form2 = class(Form)
    procedure textBox1_TextChanged(sender: Object; e: EventArgs);
    procedure button1_Click(sender: Object; e: EventArgs);
    procedure button2_Click(sender: Object; e: EventArgs);
  {$region FormDesigner}
  internal
    {$resource Unit2.Form2.resources}
    label1: &Label;
    textBox1: TextBox;
    button1: Button;
    button2: Button;
    b1: boolean;
    {$include Unit2.Form2.inc}
  {$endregion FormDesigner}
  public
    constructor;
    begin
      b1 := False;      
      InitializeComponent;
    end;
  end;
implementation
 
procedure Form2.textBox1_TextChanged(sender: Object; e: EventArgs);
begin
  if textBox1.Text = '' then
  begin
    button1.Enabled := False;
    b1 := False
  end
  else
  begin
    button1.Enabled := True;
    b1 := True
  end
end;
 
procedure Form2.button1_Click(sender: Object; e: EventArgs);
begin
  textBox1.Text := ''
end;
 
procedure Form2.button2_Click(sender: Object; e: EventArgs);
begin
  if b1 then
  begin
    Form1.listBox1.Items.Add(textBox1.Text);
    close
  end
end;
end.