Firefox extension autofill broken — InputJsonDeserializationFailed

After digging a bit : 

The error is not raised on every web site:

- error occurs on clever cloud console (https://api.clever-cloud.com/v2/sessions/login)
- error doesn't occur on github.com

What I can see is that in the next-fill event JSON data, I have a null frame on clever cloud console:

{
  "name": "next-fill",
  "data": {
    "collectedPageDetails": {
      "unparsedUrl": "https://api.clever-cloud.com/v2/sessions/login?fromAuthorize=true&partner_id=01907806-284e-1b4d-029a-20b9ebfcb8c0",
      "title": "Log in · Clever Cloud",
      "frames": [
        {
          "fields": [
            {
              "formOpid": 0,
              "htmlId": "login-email",
              "htmlName": "email",
              "htmlClass": "form-control",
              "isActive": true,
              "opid": 0,
              "tabIndex": 1,
              "type": "email",
              "dataAttributes": {},
              "label": "EMAIL ADDRESS",
              "labelBefore": "Email address"
            },
            {
              "formOpid": 0,
              "htmlId": "login-pwd",
              "htmlName": "pass",
              "htmlClass": "form-control",
              "maxLength": 4096,
              "minLength": 6,
              "opid": 1,
              "tabIndex": 2,
              "type": "password",
              "dataAttributes": {},
              "label": "PASSWORD",
              "labelBefore": "Password Lost password?"
            },
            {
              "formOpid": 0,
              "htmlClass": "btn-signup btn-github",
              "opid": 2,
              "tabIndex": 3,
              "type": "submit",
              "value": "LOG IN",
              "dataAttributes": {}
            }
          ],
          "forms": [
            {
              "htmlAction": "https://api.clever-cloud.com/v2/sessions/login",
              "htmlMethod": "post",
              "opid": 0,
              "textContent": "Email Login EMAIL ADDRESS PASSWORD LOST PASSWORD? LOG IN OR GitHub Login CONNECT"
            }
          ],
          "origin": {
            "type": "url",
            "content": {
              "origin": "https://api.clever-cloud.com",
              "pathName": "/v2/sessions/login"
            }
          },
          "title": "Log in · Clever Cloud",
          "uuid": "ka3l3x"
        },
        null
      ]
    }
  }
}

if i dynamically drop this null frame, autofill works.