FEM: Purge tool deletes all result objects. Fixes #23028

- Correctly deletes children of pipeline without error on delete
- Ensures visualizations are also deleted as they are result objects too
- Ensures purge tool uses a transaction, like all tools should

src/Mod/Fem/App/FemPostGroupExtension.cpp

@@ -109,11 +109,8 @@ App::DocumentObject* FemPostGroupExtension::getGroupOfObject(const App::Document
 
 void FemPostGroupExtension::onExtendedUnsetupObject()
 {
-    // remove all children!
-    auto document = getExtendedObject()->getDocument();
-    for (const auto& obj : Group.getValues()) {
-        document->removeObject(obj->getNameInDocument());
-    }
+    // remove all children (if not already removed)!
+    removeObjectsFromDocument();
 }
 
 bool FemPostGroupExtension::allowObject(App::DocumentObject* obj)