fix : if Flatten was the root node then the graph has no root node anymore after node deletion

src/graph/GraphView.cpp

@@ -912,6 +912,13 @@ bool Aidge::GraphView::replace(const std::shared_ptr<GraphView>& oldG, const std
                 for (std::size_t i = 0; i < oldOI.size(); ++i) {
                     if (inputParents[i].first)
                         inputParents[i].first -> addChild(outputChildren[i].first, inputParents[i].second, outputChildren[i].second);
+                    if (inputParents[i].first) {
+                      inputParents[i].first -> addChild(outputChildren[i].first, inputParents[i].second, outputChildren[i].second);
+                    } else {
+                      for (auto & view : commonGraphViews){
+                        view->setRootNode( outputChildren[i].first);
+                      }
+                    }
                 }
             }
             else if ((oldOI.size() == 1) && (inputParents[0].first)) {