How do I pass a 'null' action

Pass in an empty action if you want to:

DoExport((x, y) => { })

Second, you have to review your code, since passing in null is perfectly fine.

public void X()
{
    A(null);
}

public void A(Action<ColumnView, bool> a)
{
    if (a != null)
    {
        a();
    }
}

Or as per C# 6 (using the null-propagation operator):

public void A(Action<ColumnView, bool> a)
{
    a?.Invoke();
}

Or just handle it inside of the method:

private void DoExport(Action<ColumnView, bool> UpdateColumns)  
{  
  if (UpdateColumns != null)
    UpdateColumns(...);
}

You can pass an action that does nothing:

DoExport((_, __) => { });

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