Cannot find module 'superagent' Ionic 3

Hi to everyone. I’m developing a Ionic 3 application and from a few days, when I launch a command (such “ionic serve”, or “cordova build android”) this error appears to me.

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Despite it, all works fine.
Probably I can fix it installing this superagent module, but I don’t understand why this error appeared out of nowhere and what superagent module is.

Thank you in advance.

up, I still have the problem!