Update handle_access_token_response.dart
diff --git a/lib/src/handle_access_token_response.dart b/lib/src/handle_access_token_response.dart
index ba78d94..83d57fa 100644
--- a/lib/src/handle_access_token_response.dart
+++ b/lib/src/handle_access_token_response.dart
@@ -71,7 +71,7 @@
     if (expiresIn != null) {
       if (expiresIn is String) {
         try {
-          expiresIn = double.tryParse(expiresIn).toInt();
+          expiresIn = double.tryParse(expiresIn)!.toInt();
         } catch (e) {
           throw FormatException(
               'parameter "expires_in" was not an int, and given string cant not be parsed to int : "$expiresIn"');