Which method/ class is throwing this error message?

Question

When I try to reach the backend of the extension I made for TYPO3 v9.5.9, I get this error message:

To fix it, I want to find out which class/ method is throwing this error message. Does anyone know how I can find that out?

my composer.json file looks like this:

{
    "name": "secsign/secsign",
    "type": "typo3-cms-extension",
    "description": "This extension allows users to authenticate using their smart phone running the SecSign App.",
    "authors": [
        {
            "name": "SecSign Technologies Inc.",
            "role": "Developer"
        }
    ],
    "require": {
        "typo3/cms-core": "^9.5"
    },
    "autoload": {
        "psr-4": {
            "Secsign\\Secsign\\": "Classes"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Secsign\\Secsign\\Tests\\": "Tests"
        }
    },
    "replace": {
        "secsign/secsign": "self.version",
        "typo3-ter/secsign": "self.version"
    }
}

Answer 1

make sure you added something like this in your composer.json (when the file is from a custom ext, not 3rd party)

{
    // ...
    "autoload": {
        "psr-4": {
            "Secsign\\Secsign\\": "web/typo3conf/ext/SECSIGNEXT/Classes/",
        }
    }
    // ...
}