Error while "Refreshing" custom connector in Power BI Service

Hi, I made a custom connector for QuickBooks Online(Because Power BI default connector was not returning values from "Invoice" and Invoice related tables). I am able to get the data and publish in Power BI Desktop. I have included it in the enterprise gateway and able to see my report in Power BI Service, but on refresh from service I am recieving this error.
Underlying Error Message: "Value cannot be null: Parameter name: access Token Table: Invoice"

DM_ErrorDetailNameCode_UnderlyingHResult: -2147467261
I am pasting my OAuth Codes for reference.

// Data Source Kind description
PBIServiceRefresh = [
    TestConnection = (dataSourcePath) => { "PBIServiceRefresh.Contents" },
    Authentication = [
        OAuth = [
            StartLogin = StartLogin,
            FinishLogin = FinishLogin,
            Refresh = Refresh,
            Logout = Logout
        ]
    ],
    Label = "QBO Invoice Connector"    
];

// StartLogin function definition
StartLogin = (resourceUrl, state, display) =>
    let
        authorizeUrl = authorize_uri & Uri.BuildQueryString([
            client_id = client_id,  
            redirect_uri = redirect_uri,
            state = state,
            scope = "com.intuit.quickbooks.accounting",
            response_type = "code",
            response_mode = "query",
            login = "login"    
        ])
    in
        [
            LoginUri = authorizeUrl,
            CallbackUri = redirect_uri,
            WindowHeight = 720,
            WindowWidth = 1024,
            Context = null
        ];

// FinishLogin function definition
FinishLogin = (context, callbackUri, state) =>
    let
        parts = Uri.Parts(callbackUri)[Query],
        result = if (Record.HasFields(parts, {"error", "error_description"})) then 
                    error Error.Record(parts[error], parts[error_description], parts)
                 else
                    TokenMethod("authorization_code", "code", parts[code])
    in
        result;

// Refresh and Logout functions
Refresh = (resourceUrl, refresh_token) => TokenMethod("refresh_token", "refresh_token", refresh_token);
Logout = (token) => logout_uri;

// TokenMethod definition
TokenMethod = (grantType, tokenField, code) =>
    let
        queryString = [
            grant_type = "authorization_code",
            redirect_uri = redirect_uri,
            client_id = client_id,
            client_secret = client_secret
        ],
        queryWithCode = Record.AddField(queryString, tokenField, code),
        tokenResponse = Web.Contents(token_uri, [
            Content = Text.ToBinary(Uri.BuildQueryString(queryWithCode)),
            Headers = [
                #"Content-type" = "application/x-www-form-urlencoded",
                #"Accept" = "application/json"
            ],
            ManualStatusHandling = {400} 
        ]),
        body = Json.Document(tokenResponse),
        result = if (Record.HasFields(body, {"error", "error_description"})) then 
                    error Error.Record(body[error], body[error_description], body)
                 else
                    body
    in
        result;

Using this connector I am able to refresh in PowerBI Desktop.
Can anyone tell me what could be the problem?

Thanks in advance.