WPF MemberwiseClone() causes treeview binding error

I have a 3-layer treeview defined in XAML and initially when the tree has only 1 root node and no child nodes, it is working fine. When I try to edit properties in the root node, I use MemberwiseClone to make a back up root node for editing as editing the root node will not affect the child node information. The problem is after the editing is done, when the back up root node uses MemberwiseClone to copy the new root node information back to the original root node, I get the following error:

System.Windows.Data Error: 39 : BindingExpression path error: 'm_EpVMList' property not found on 'object' ''MainViewModel' (HashCode=35912612)'. BindingExpression:Path=m_EpVMList; DataItem='MainViewModel' (HashCode=35912612); target element is 'TreeViewItem' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')

This is very strange as m_EpVMListindeed is not a property of MainViewModel, but a subitem list of the main node which is an ObservationCollection list in the MainViewModel. Here is the code:

<TreeView x:Name="MainTree" ItemsSource="{Binding m_NodeList}" 
    SelectedItemChanged="MainTree_SelectedItemChanged"><TreeView.Resources><HierarchicalDataTemplate DataType="{x:Type vm:NodeViewModel}" ItemsSource="{Binding Path=m_EpVMList}"  ><StackPanel><TextBlock  Text="{Binding Path=DisplayName}" /></StackPanel></HierarchicalDataTemplate><HierarchicalDataTemplate DataType="{x:Type vm:EpViewModel}" ItemsSource="{Binding Path=IntfViewModel}" ><StackPanel Orientation="Horizontal"  Margin="2"><TextBlock  Text="{Binding Path=DisplayName}" /></StackPanel></HierarchicalDataTemplate><HierarchicalDataTemplate DataType="{x:Type vm:IntfViewModel}" ><StackPanel><TextBlock  Text="{Binding Path=DisplayName}"/></StackPanel></HierarchicalDataTemplate></TreeView.Resources></TreeView>

The XAML's datacontext has been assigned to MainViewModel:

//in MainViewModel.cs

 private ObservableCollection<NodeViewModel> _m_NodeList = new ObservableCollection<NodeViewModel>();
 public ObservableCollection<NodeViewModel> m_NodeList
 {
       get { return _m_NodeList; }
       set
       {
          if (value != _m_NodeList)
          {
             _m_NodeList = value;
             Notify("m_NodeList"); //For OnPropertyChange
          }
       }
  }

//in NodeViewModel.cs
 private ObservableCollection<EpViewModel> _m_EpVMList = new ObservableCollection<EpViewModel>();
public ObservableCollection<EpViewModel> m_EpVMList
{
    get { return _m_EpVMList; }
    set
    {
        if (_m_EpVMList != value)
        {
            _m_EpVMList = value;
            Notify("m_EpVMList");
        }
    }
}

It seems try to look for m_EpVMList in MainViewModel which should actually be in NodeViewModel. However, this error will not appear when the first time the tree is loaded.

Is there a way to solve this error? I thought about adding DataContext in each HierarchicalDataTemplate in treeview. But not sure if it will solve the issue and how to do it exactly.

Any help is greatly appreciated.

Angela