To reproduce: 
1. Add UserControl
2. Drag and drop RadPopupEditor and RadPopupContainer
3. Open the smart tag of RadPopupEditor and try to set the AssociatedControl property. You will see error message from attached image. 

Workaround: 
Set the AssociatedControl at run time. Here is the code snippet: 

For C#: 
public UserControl1()
{
    InitializeComponent();
    this.radPopupEditor1.AssociatedControl = this.radPopupContainer1;
}

For VB:
Public Class UserControl1
    Inherits UserControl
    Public Sub New()
        InitializeComponent()
        Me.RadPopupEditor1.AssociatedControl = Me.RadPopupContainer1
    End Sub
End Class