Hmac sha1 and base64 encoding in dart

I am trying to produce oauth_signature to use Fatsecret API, but getting an invalid signature error.

To create oauth_signature value the Documentation says:

Use the HMAC-SHA1 signature algorithm as defined by the [RFC2104] to sign the request where text is the Signature Base String and key is the concatenated values of the Consumer Secret and Access Secret separated by a '&' character (show '&' even if Access Secret is empty as some methods do not require an Access Token).
The calculated digest octet string, first base64-encoded per [RFC2045], then escaped using the [RFC3986] percent-encoding (%xx) mechanism is the oauth_signature.

Following code is used to generate the signature

String _generateSignature(String method, String url, Map<String, String> params) {
// sort the parameters
var sortedParams = SplayTreeMap.from(params);

// Concatenate the sortedParams with '&'
String concatenatedParams = sortedParams.keys.map((key) {
  return '$key=${sortedParams[key]}';
}).join('&');

// encode the sorted and concatenated params string
var encodedParams = Uri.encodeComponent(concatenatedParams);

var encodedUrl = Uri.encodeComponent(url);

String baseString = '$method&$encodedUrl&$encodedParams';
print(baseString);

String signingKey = '${Uri.encodeComponent(SHARED_SECRET)}&';

var hmac = Hmac(sha1, signingKey.codeUnits);

return base64Encode(hmac.convert(baseString.codeUnits).bytes);
}

When I make API call, it returns

11-12 09:52:45.924 15525-15612/com.example.delete I/flutter: {error: {code: 8, message: Invalid signature: oauth_signature 'y81+JIzX/P+xNqOCYLgbrMtDV2I='}}

Please help me, guys...I am stuck at this for weeks!

If i modify the code like below then i am not getting any Invalid signature error.

String _generateSignature(String method, String url, Map<String, dynamic> params) {
    // Sort the parameters
    var sortedParams = SplayTreeMap.from(params);

    // Concatenate the sortedParams with '&'
    String concatenatedParams = sortedParams.keys.map((key) {
      return '$key=${sortedParams[key]}';
    }).join('&');

    // encode the sorted and concatenated params string
    var encodedParams = Uri.encodeComponent(concatenatedParams);

    var encodedUrl = Uri.encodeComponent(url);

    String baseString = '$method&$encodedUrl&$encodedParams';

    String signingKey = '${Uri.encodeComponent(SHARED_SECRET)}&';

    var hmac = Hmac(sha1, signingKey.codeUnits);

    return Uri.encodeComponent(base64Encode(hmac.convert(baseString.codeUnits).bytes));
  }