Fixed critical issue with Node::views()

include/aidge/graph/Node.hpp

@@ -252,7 +252,9 @@ public:
   inline std::set<std::shared_ptr<GraphView>> views() const noexcept {
     std::set<std::shared_ptr<GraphView>> res;
     for (const auto &v : mViews) {
-      res.insert(v.lock());
+      if (auto p = v.lock()) {
+        res.insert(p);
+      }
     }
     return res;
   }