RadDock cause wrong Form's ActiveControl

HI

I have met a problem about WinForm ActiveControl in Telerik Control - RadDock.

Reintroduce steps :

1.Place TextBox1, TextBox2 on Form.
2a.Place a RadDock on Form.
2b.RadDock task - Dock New Window to Left - toolWindow1
2c.RadDock task - Add Tabbed Document - documentWindow1
3a.Place TextBox3 on toolWindow1.
3b.Place TextBox4 on documentWindow1.

Run the application and test the ActiveControl state (focus the control by control),
ActiveControl of TextBox1 and TextBox2 is CORRECT,
but ActiveControl of TextBox3 and TextBox4 was WRONG.
(see attachment - Telerik ActiveControl err1.png).

Why the ActiveControl are not TextBox3/TextBox4 ? 
And Is there have any solution to fix that ?

Code :


    private void Timer1_Tick(object sender, EventArgs e)
    {
      Control CurrentActiveControl = ActiveControl;
      string sActiveControlName, sActiveControlString;
      //
      sActiveControlName = (CurrentActiveControl != null) ? CurrentActiveControl.Name : "(unknown)";
      sActiveControlString = (CurrentActiveControl != null) ? CurrentActiveControl.ToString() : "(none)";
      //
      listBox1.Items.Insert(0, DateTime.Now.ToString("hh:mm:ss - ") + sActiveControlName + ", " + sActiveControlString);
    }

*Visual Studio 2015 Enterprise Update 3
*Telerik UI for WinForms R1 2018

Best regards

Chris