Vielen Dank an euch:
Ich versuche das Script auch so umzusetzen:

' API-Deklarationen
Declare PtrSafe Function FindWindow Lib "user32" Alias "FindWindowA" ( _
    ByVal lpClassName As String, _
    ByVal lpWindowName As String) As Long

Declare PtrSafe Function FindWindowEx Lib "user32" Alias "FindWindowExA" ( _
    ByVal hWndParent As Long, _
    ByVal hWndChildAfter As Long, _
    ByVal lpszClass As String, _
    ByVal lpszWindow As String) As Long

Declare PtrSafe Function SendMessage Lib "user32" Alias "SendMessageA" ( _
    ByVal hWnd As Long, _
    ByVal Msg As Long, _
    ByVal wParam As Long, _
    ByRef lParam As Any) As Long

Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" ( _
    ByRef Destination As Any, _
    ByRef Source As Any, _
    ByVal Length As Long)
    
Declare PtrSafe Function GetClassNameA Lib "user32" ( _
    ByVal hWnd As Long, _
    ByVal lpClassName As String, _
    ByVal nMaxCount As Long) As Long

' Konstanten für TreeView-Operationen
Const TV_FIRST As Long = &H1100
Const TVM_GETNEXTITEM As Long = TV_FIRST + 10
Const TVM_GETITEM As Long = TV_FIRST + 12
Const TVGN_ROOT As Long = &H0
Const TVGN_NEXT As Long = &H1
Const TVIF_TEXT As Long = &H1

' TreeView-Item Struktur
Type tvItem
    mask As Long
    hItem As Long
    state As Long
    stateMask As Long
    pszText As LongPtr
    cchTextMax As Long
    iImage As Long
    iSelectedImage As Long
    cChildren As Long
    lParam As Long
End Type

Sub EnumerateTreeView()
    Dim hWndParent As Long
    Dim hWndTree As Long
    Dim hItem As Long
    Dim tvItem As tvItem
    Dim buffer As String
    Dim result As Long
    Dim row As Integer

    ' Fenster "Diff" suchen
    hWndParent = FindWindowHandleByTitle("DIFF")
    If hWndParent = 0 Then
        MsgBox "Fenster mit Titel 'Diff' nicht gefunden!"
        Exit Sub
    End If

    ' Suche nach dem TreeView-Steuerelement im Fenster
    hWndTree = FindChildWindowHandleByClassName(hWndParent, "SysTreeView32")
    If hWndTree = 0 Then
        MsgBox "TreeView-Steuerelement nicht gefunden!"
        Exit Sub
    End If

    ' Root-Knoten abrufen
    hItem = SendMessage(hWndTree, TVM_GETNEXTITEM, TVGN_ROOT, ByVal 0&) '=> Hier stürzt das Programm ab.
    If hItem = 0 Then
        MsgBox "Kein Root-Knoten gefunden."
        Exit Sub
    End If

    row = 1

    Do While hItem <> 0
        buffer = String(256, vbNullChar) ' Puffer initialisieren

        ' TVITEM initialisieren
        With tvItem
            .mask = TVIF_TEXT
            .hItem = hItem
            .pszText = StrPtr(buffer)
            .cchTextMax = Len(buffer)
        End With

        ' Knoteninformationen abrufen
        result = SendMessage(hWndTree, TVM_GETITEM, 0, tvItem)
        If result <> 0 Then
            ' Knoteninformationen in die Tabelle schreiben
            Cells(row, 5).Value = Left(buffer, InStr(buffer, vbNullChar) - 1)
            row = row + 1
        Else
            Debug.Print "Fehler beim Abrufen des Knotens: " & hItem
        End If

        ' Nächsten Knoten abrufen
        hItem = SendMessage(hWndTree, TVM_GETNEXTITEM, TVGN_NEXT, hItem)
    Loop

    MsgBox "Fertig! Knoten wurden ab Spalte E eingefügt."
End Sub

Function FindWindowHandleByTitle(windowTitle As String) As Long
    ' Sucht nach einem Fenster mit einem bestimmten Titel und gibt das Handle zurück
    FindWindowHandleByTitle = FindWindow(vbNullString, windowTitle)
End Function

Function FindChildWindowHandleByClassName(hWndParent As Long, className As String) As Long
    Dim hWndChild As Long
    hWndChild = FindWindowEx(hWndParent, 0, vbNullString, vbNullString)

    Do While hWndChild <> 0
        ' Prüfen, ob das aktuelle Fenster die gewünschte Klasse hat
        If GetWindowClassName(hWndChild) = className Then
            FindChildWindowHandleByClassName = hWndChild
            Exit Function
        End If

        ' Rekursiv nach weiteren Kindern suchen
        FindChildWindowHandleByClassName = FindChildWindowHandleByClassName(hWndChild, className)
        If FindChildWindowHandleByClassName <> 0 Then Exit Function

        ' Nächstes Kind suchen
        hWndChild = FindWindowEx(hWndParent, hWndChild, vbNullString, vbNullString)
    Loop

    ' Kein passendes Fenster gefunden
    FindChildWindowHandleByClassName = 0
End Function

Function GetWindowClassName(hWnd As Long) As String
    Dim classNameBuffer As String
    classNameBuffer = String(256, vbNullChar)
    Dim result As Long
    result = GetClassNameA(hWnd, classNameBuffer, Len(classNameBuffer))
    If result <> 0 Then
        GetWindowClassName = Left(classNameBuffer, result)
        Debug.Print GetWindowClassName
    Else
        GetWindowClassName = vbNullString
    End If
End Function

Nur stürzt mein externes Programm ab beim Zugriff mit "SendMessage"