Adds fixes to the grammar and rtc session (#317)

* Added default response to cancellation

* Added flag to stop ConcurrentModificationError when stopping ua

* update.

* Fixed grammar

* Added default reason to cancellation

Co-authored-by: Perondas <[email protected]>
Co-authored-by: Perondas <[email protected]>
Co-authored-by: cloudwebrtc <[email protected]>

Author: 4 people

lib/src/grammar_parser.dart

@@ -17187,7 +17187,7 @@ class GrammarParser {
             _failure(_expect83);
           }
           if (success) {
-            final $1 = $$;
+            final $1 = $$[2];
             final $start = startPos1;
             var pos0 = _startPos;
             $$ = ((offset, expires) {
@@ -17492,7 +17492,7 @@ class GrammarParser {
     if (!success && _cursor > _testing) {
       _failure(_expect114);
     }
-    return $$;
+    return data;
   }
 
   dynamic _parse_substate_value() {
@@ -22477,7 +22477,7 @@ class GrammarParser {
     if (!success && _cursor > _testing) {
       _failure(_expect113);
     }
-    return $$;
+    return data;
   }
 
   dynamic parse_Supported() {

lib/src/rtc_session.dart

@@ -759,7 +759,7 @@ class RTCSession extends EventManager implements Owner {
           _is_canceled = true;
           _cancel_reason = cancel_reason;
         } else if (_status == C.STATUS_1XX_RECEIVED) {
-          _request.cancel(cancel_reason);
+          _request.cancel(cancel_reason ?? '');
         }
 
         _status = C.STATUS_CANCELED;