MemoryListener: checking for nullptr

source/RobotAPI/libraries/armem/client/util/MemoryListener.h

@@ -63,12 +63,16 @@ namespace armarx::armem::client::util
             };
             subscribe(subscriptionID, cb);
         }
+
         template <class CalleeT>
         void subscribe(const MemoryID& subscriptionID, CalleeT* callee, MemberCallbackUpdatedOnly<CalleeT> callback)
         {
             auto cb = [callee, callback](const MemoryID&, const std::vector<MemoryID>& updatedSnapshotIDs)
             {
-                (callee->*callback)(updatedSnapshotIDs);
+                if(callee)
+                {
+                    (callee->*callback)(updatedSnapshotIDs);
+                }
             };
             subscribe(subscriptionID, cb);
         }