Underlying error message:Value cannot be null. Parameter name: uriString

// This file contains your Data Connector logic
section GitHub;

redirect_uri = "https://oauth.powerbi.com/views/oauthredirect.html";
client_id = "";
client_secret = "";
windowWidth = 1000;
windowHeight = 800;


[DataSource.Kind="GitHub", Publish="GitHub.Publish"]
shared GitHub.Contents = (optional message as text) =>
    let
        _message = if (message <> null) then message else "(no message)",
        a = "Hello from GitHub: " & _message
    in
        a;

// Data Source Kind description
GitHub = [
    Authentication = [
        OAuth = [
            StartLogin = StartLogin,
            FinishLogin = FinishLogin
        ]
    ],
    Label = Extension.LoadString("DataSourceLabel")
];

StartLogin = (resourceUrl, state, display) =>
        let
            TestConnection = (url) => {"GitHub.Contents"},
            AuthorizeUrl = "https://Github.com/login/oauth/authorize?" & Uri.BuildQueryString([
                client_id = client_id,
                scope = "user, repo",
                state = state,
                redirect_uri = redirect_uri])
        in
            [
                LoginUri = AuthorizeUrl,
                CallbackUri = redirect_uri,
                WindowHeight = windowHeight,
                WindowWidth = windowWidth,
                Context = null
            ];

FinishLogin = (context, callbackUri, state) =>
    let
        Parts = Uri.Parts(callbackUri)[Query]
    in
        TokenMethod(Parts[code]);

 TokenMethod = (code) =>
    let
        Response = Web.Contents("https://Github.com/login/oauth/access_token", [
            Content = Text.ToBinary(Uri.BuildQueryString([
                client_id = client_id,
                client_secret = client_secret,
                code = code,
                redirect_uri = redirect_uri])),
            Headers=[#"Content-type" = "application/x-www-form-urlencoded",#"Accept" = "application/json"]]),
        Parts = Json.Document(Response)
    in
        Parts;

// Data Source UI publishing description
GitHub.Publish = [
    Beta = true,
    Category = "Other",
    ButtonText = { Extension.LoadString("ButtonTitle"), Extension.LoadString("ButtonHelp") },
    LearnMoreUrl = "https://powerbi.microsoft.com/",
    SourceImage = GitHub.Icons,
    SourceTypeImage = GitHub.Icons
];

GitHub.Icons = [
    Icon16 = { Extension.Contents("GitHub16.png"), Extension.Contents("GitHub20.png"), Extension.Contents("GitHub24.png"), Extension.Contents("GitHub32.png") },
    Icon32 = { Extension.Contents("GitHub32.png"), Extension.Contents("GitHub40.png"), Extension.Contents("GitHub48.png"), Extension.Contents("GitHub64.png") }
];