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Hello, @AbbasSidaoui . I think this is too late to help you, but I hope to help someone like me, who googled with the error message.
The main cause of this strange error message is there is currently no error handler for partial/multiple/broken json data. This is mentioned in source code as a comment.
https://github.com/RobotWebTools/rosbridge_suite/blob/c78381703d899cd491f14eebc0511bc2850e49ae/rosbridge_library/src/rosbridge_library/protocol.py#L140-L151
So, any partial/multiple/broken json data' error is catched by this part and this causes the error message:"Received a message without op."
https://github.com/RobotWebTools/rosbridge_suite/blob/c78381703d899cd491f14eebc0511bc2850e49ae/rosbridge_library/src/rosbridge_library/protocol.py#L180-L185
In your case, I can see your typo in your screenshot, "frame_id" = "map". I think this was the cause.
To determine where is the typo, I recommend to put your message into json.loads() in json library of python since it is used by rosbridge_library to parse JSON.
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