c# - winform アプリを Mac 上の monodevelop に移植する

Question

Visual Studio で作成した winforms アプリを monodevelop で Mac に移植しようとしています。コンパイル エラーを解決した後、プログラムを実行すると、アプリケーションは次の例外を出力します。

System.NullReferenceException: Object reference not set to an instance of an object
 at System.Windows.Forms.TextBoxBase.set_Lines (System.String[] value) [0x00000] in <filename unknown>:0

  at (wrapper remoting-invoke-with-check) System.Windows.Forms.TextBoxBase:set_Lines (string[])

  at WindowsFormsApplication1.Main_Form.amp_portante_ValueChanged (System.Object Sender) [0x00025] in /Users/giuseppe/Desktop/CRUZER/Csound Converter/WindowsFormsApplication1/Main_Form.cs:381

  at KnobControl.KnobControl.OnValueChanged (System.Object sender) [0x00000] in <filename unknown>:0

  at KnobControl.KnobControl.set_Value (Int32 value) [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) KnobControl.KnobControl:set_Value (int)
  at WindowsFormsApplication1.Main_Form.textb2_TextChanged (System.Object sender, System.EventArgs e) [0x00085] in /Users/giuseppe/Desktop/CRUZER/Csound Converter/WindowsFormsApplication1/Main_Form.cs:1023 
  at System.Windows.Forms.Control.OnTextChanged (System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.TextBoxBase.OnTextChanged (System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.TextBoxBase.set_Text (System.String value) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.TextBox.set_Text (System.String value) [0x00000] in <filename unknown>:0 
  at WindowsFormsApplication1.Main_Form.SimpleFM_Conf () [0x00058] in /Users/giuseppe/Desktop/CRUZER/Csound Converter/WindowsFormsApplication1/Main_Form.cs:45 
  at WindowsFormsApplication1.Main_Form.Main_Form_Load (System.Object sender, System.EventArgs e) [0x00012] in /Users/giuseppe/Desktop/CRUZER/Csound Converter/WindowsFormsApplication1/Main_Form.cs:359 
  at System.Windows.Forms.Form.OnLoad (System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Form.OnLoadInternal (System.EventArgs e) [0x00000] in <filename unknown>:0

アプリケーションは Visual Studio で完全に動作しますが、何をすべきかわかりません。