Null values in nodes descriptions on a treeview

Guys

I wonder if it's possible to sort out nulls among items which I display as a node description.
In this case a have few ABC Indicators which are null so I'd avoid changing their value to N/A and add some if statement to omit them. I tried to equal it to null and iserror but no luck.
As you can see below I tried Nz() as well

                            If Nz(CStr(PS![ABC Indicator])) = "A" Then
                                Set a = TreeView.Nodes.Add(CStr(PS![Superior Function Location]), tvwChild, CStr(PS![Functional location]), CStr(PS![Functional location]) & "     (" & CStr(PS![Function Location Description]) & ")" & "     (" & Nz(CStr(PS![ABC Indicator])) & ")" & "     (" & CStr(PS![Object type]) & ")" & "     (" & CStr(PS![Object Type Desc]) & ")")
                                a.ForeColor = RGB(63, 23, 127) 'violet
                            ElseIf Nz(CStr(PS![ABC Indicator])) = "B" Then
                                Set a = TreeView.Nodes.Add(CStr(PS![Superior Function Location]), tvwChild, CStr(PS![Functional location]), CStr(PS![Functional location]) & "     (" & CStr(PS![Function Location Description]) & ")" & "     (" & Nz(CStr(PS![ABC Indicator])) & ")" & "     (" & CStr(PS![Object type]) & ")" & "     (" & CStr(PS![Object Type Desc]) & ")")
                                a.ForeColor = RGB(255, 97, 3) 'orange
                            ElseIf Nz(CStr(PS![ABC Indicator])) = "C" Then
                                Set a = TreeView.Nodes.Add(CStr(PS![Superior Function Location]), tvwChild, CStr(PS![Functional location]), CStr(PS![Functional location]) & "     (" & CStr(PS![Function Location Description]) & ")" & "     (" & Nz(CStr(PS![ABC Indicator])) & ")" & "     (" & CStr(PS![Object type]) & ")" & "     (" & CStr(PS![Object Type Desc]) & ")")
                                a.ForeColor = RGB(24, 116, 205) 'blue
                            Else
                                Set a = TreeView.Nodes.Add(CStr(PS![Superior Function Location]), tvwChild, CStr(PS![Functional location]), CStr(PS![Functional location]) & "     (" & CStr(PS![Function Location Description]) & ")" & "     (" & CStr(PS![Object type]) & ")" & "     (" & CStr(PS![Object Type Desc]) & ")")
                            End If

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