more meaningful warning

source/RobotAPI/libraries/armem_robot_state/client/common/RobotReader.cpp

@@ -117,7 +117,7 @@ namespace armarx::armem::robot_state
 
         if (not memoryReader)
         {
-            ARMARX_WARNING << "Memory reader is null";
+            ARMARX_WARNING << "Memory reader is null. Did you forget to call RobotReader::connect() in onConnectComponent()?";
             return std::nullopt;
         }