Correct the return value of wxDataViewTreeCtrl::GetNthChild()

Wrong pointer was used to initialize wxDataViewItem (apparently since
always, i.e. this function could never have been used successfully...).

Closes #18132.

src/common/datavcmn.cpp

@@ -2619,7 +2619,7 @@ wxDataViewItem wxDataViewTreeStore::GetNthChild( const wxDataViewItem& parent, u
 
     wxDataViewTreeStoreNode* const node = parent_node->GetChildren()[pos];
     if (node)
-        return wxDataViewItem(node->GetData());
+        return node->GetItem();
 
     return wxDataViewItem(0);
 }