address comments

src/objective-c/GRPCClient/GRPCInterceptor.h

@@ -74,7 +74,7 @@
  *                                  receiveNextMessages
  *                               didReceiveInitialMetadata
  *                                    didReceiveData
- *                                     didWriteData
+ *                                     didWriteData                   receiveNextmessages
  *           writeData  -----             -----                 ----  didReceiveInitialMetadata
  * receiveNextMessages |     |           |     |               |    | didReceiveData
  *                     |     V           |     V               |    V didWriteData

src/objective-c/GRPCClient/GRPCInterceptor.m

@@ -44,40 +44,50 @@
 
 - (void)startNextInterceptorWithRequest:(GRPCRequestOptions *)requestOptions
                             callOptions:(GRPCCallOptions *)callOptions {
+  if (_nextInterceptor != nil) {
     id<GRPCInterceptorInterface> copiedNextInterceptor = _nextInterceptor;
     dispatch_async(copiedNextInterceptor.requestDispatchQueue, ^{
       [copiedNextInterceptor startWithRequestOptions:requestOptions callOptions:callOptions];
     });
   }
+}
 
 - (void)writeNextInterceptorWithData:(id)data {
+  if (_nextInterceptor != nil) {
     id<GRPCInterceptorInterface> copiedNextInterceptor = _nextInterceptor;
     dispatch_async(copiedNextInterceptor.requestDispatchQueue, ^{
       [copiedNextInterceptor writeData:data];
     });
   }
+}
 
 - (void)finishNextInterceptor {
+  if (_nextInterceptor != nil) {
     id<GRPCInterceptorInterface> copiedNextInterceptor = _nextInterceptor;
     dispatch_async(copiedNextInterceptor.requestDispatchQueue, ^{
       [copiedNextInterceptor finish];
     });
   }
+}
 
 - (void)cancelNextInterceptor {
+  if (_nextInterceptor != nil) {
     id<GRPCInterceptorInterface> copiedNextInterceptor = _nextInterceptor;
     dispatch_async(copiedNextInterceptor.requestDispatchQueue, ^{
       [copiedNextInterceptor cancel];
     });
   }
+}
 
 /** Notify the next interceptor in the chain to receive more messages */
 - (void)receiveNextInterceptorMessages:(NSUInteger)numberOfMessages {
+  if (_nextInterceptor != nil) {
     id<GRPCInterceptorInterface> copiedNextInterceptor = _nextInterceptor;
     dispatch_async(copiedNextInterceptor.requestDispatchQueue, ^{
       [copiedNextInterceptor receiveNextMessages:numberOfMessages];
     });
   }
+}
 
 // Methods to forward GRPCResponseHandler callbacks to the previous object