(fix) check if nextInvocation is null in exception

closes awslabs#23

lib/runtime/runtime.dart

@@ -104,17 +104,20 @@ class Runtime<T> {
         final context = Context.fromNextInvocation(nextInvocation);
 
         final func = _handlers[context.handler];
-        if(func == null) {
-          throw RuntimeException('No handler with name "${context.handler}" registered in runtime!');
+        if (func == null) {
+          throw RuntimeException(
+              'No handler with name "${context.handler}" registered in runtime!');
         }
         final event =
             Event.fromHandler(func.type, await nextInvocation.response);
         final result = await func.handler(context, event);
 
         await _client.postInvocationResponse(result);
       } on Exception catch (error, stacktrace) {
-        await _client.postInvocationError(
-            nextInvocation.requestId, InvocationError(error, stacktrace));
+        if (nextInvocation != null) {
+          await _client.postInvocationError(
+              nextInvocation.requestId, InvocationError(error, stacktrace));
+        }
       }
 
       nextInvocation = null;