Simple fetch from API in Nodejs but UnhandledPromiseRejectionWarning: nestedFunction is not a Function

I'm a non-professional using nodejs server (backend) and javascript/html (frontend) to fetch data from two API's: one API gives a response and I use an ID from the first API to fetch data from the other API. The API returns XML so I use XML2Json and JSON.parse to get the Javascript Object.

• everything works perfect until I get to the "return nestedFunction(new_details")-function in the second API-call
• so this is where the results are sent back to the client
• I do it for the first API and it works fine (backend + frontend)
• I tried Async/await but the problem isn't solved
• I get the error: "UnhandledPromiseRejectionWarning: TypeError: nestedFunction is not a function"

What could be the problem?

app.get('/AATGetTermMatch', function(req,res) {
    let termMatch = req.query.term;
    let termLogop = req.query.logop;
    let termNotes = req.query.notes;
    AATGetTermMatch(termMatch, termLogop, termNotes, function (conceptResults) {
        res.send(conceptResults); 
    });
});
function AATGetTermMatch(termMatch, termLogop, termNotes,  nestedFunction) {
    let URL = baseURL + "AATGetTermMatch?term=" + termMatch + "&logop=" + termLogop + "&notes=" + termNotes;
    fetch(URL)
        .then(function (response){ 
            return response.text(); 
        }).then(function (response) { 
        APICallResults = response;
        parseJson(); 
        let objectAPI = JSON.parse(APICallResults);
        let full_Concepts = objectAPI.Vocabulary.Subject;
        let i;
        for (i = 0; i < full_Concepts.length; i++) {
            global.ID = full_Concepts[i].Subject_ID;
            searchTermDetails(global.ID);
        } return nestedFunction(full_Concepts);
    });
}
app.get('/subjectID', function(req, res) {
    let selectedID = req.query.subjectID;
    searchTermDetails(selectedID, function (termDetails) {
        res.json(termDetails);
    });
});
2nd API : http://vocabsservices.getty.edu/AATService.asmx/AATGetSubject?subjectID=300004838
function searchTermDetails(selectedID, nestedFunction) {
    selectedID = global.ID;
    let URL_Details = baseURL + "AATGetSubject?" + "subjectID=" + selectedID;
    fetch(URL_Details)
        .then(function (response) {
            return response.text();
        }).then(function (response) {
        APICallResults_new = response;
        parseJsonAgain();
        let detailAPI = JSON.parse(APICallResults_new);
        let new_details = detailAPI.Vocabulary.Subject;
        let Subject_ID = new_details[0].Subject_ID;
        let descriptive_Notes_English = new_details[0].Descriptive_Notes[0].Descriptive_Note[0].Note_Text;
        } **return nestedFunction(new_details);**
    }).catch(function (error) {
        console.log("error");
    });
}
function parseJson() {
    xml2js.parseString(APICallResults, {object: true, trim:true, sanitize: true, arrayNotation: true, mergeAttrs: true}, (err, result) => {
        if (err) {
            throw err;
        }
        const resultJson = JSON.stringify(result, null, 4);
        //JSON.parse(resultJson);
        APICallResults = resultJson;
    });
}

function parseJsonAgain() {
    xml2js.parseString(APICallResults_new, {object: true, trim:true, sanitize: true, arrayNotation: true, mergeAttrs: true}, (err, result) => {
        if (err) {
            throw err;
        }
        const resultJsonAgain = JSON.stringify(result, null, 4);
        APICallResults_new = resultJsonAgain;
        //console.log(APICallResults_new);
    });
}

I've read many threads about this error but the proposed solutions don't seem to work.