[Flutter] Unhandled Exception: FormatException: Unexpected character (at character 1) - Flutter

I know there's another question related to this but badly ask. (Flutter: Unhandled Exception: FormatException: Unexpected character (at character 1)).

My debug console prints:

When the button is pressed throws the exception:

As you can see in the API, there's no response so there isn't communication between them both. I suppose is because of the exception.

With respect to the backend:

POST REQUEST:

GET REQUEST:

In this file I have the config of my frontend to connect with the backend:

In case you wanna verify that my IP is correct:

Get this after running ifconfig in an Ubuntu OS:

Really hope that anyone can help me out, thanks in advance !!

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