Dockpanel not working in inherited form

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C#

WinForms

DockPanel

I m working on project in which I used One Main Form and other forms are inherited from Main form through DigitalRune.Windows.Docking Dock Panel . now I want to go to next form through this dockpanel which is also inherited from Main Form

private void btnNext_Click(object sender, EventArgs e)
        {
                if (fSecond == null)
                {
                    fSecond = new frmSecondLecture();
                    fSecond.Show(dp, DigitalRune.Windows.Docking.DockState.Document);
                }
                else
                {
                    fSecond.Activate();
                }

I set name of dock panel dp in above code. Now it show me error on dp in following line

fSecond.Show(dp, DigitalRune.Windows.Docking.DockState.Document);

It show me that dp is not exist in current context. What can I do now?