fix some threading issues

src/objective-c/GRPCClient/GRPCCall.m

@@ -468,7 +468,9 @@ const char *kCFStreamVarName = "grpc_cfstream";
 
 - (void)cancelCall {
   // Can be called from any thread, any number of times.
-  [_wrappedCall cancel];
+  @synchronized (self) {
+    [_wrappedCall cancel];
+  }
 }
 
 - (void)cancel {
@@ -730,17 +732,21 @@ const char *kCFStreamVarName = "grpc_cfstream";
   _responseWriteable =
       [[GRXConcurrentWriteable alloc] initWithWriteable:writeable dispatchQueue:_responseQueue];
 
-  _wrappedCall = [[GRPCWrappedCall alloc] initWithHost:_host path:_path callOptions:_callOptions];
-  if (_wrappedCall == nil) {
+  GRPCWrappedCall *wrappedCall = [[GRPCWrappedCall alloc] initWithHost:_host path:_path callOptions:_callOptions];
+  if (wrappedCall == nil) {
     [self maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
                                                    code:GRPCErrorCodeUnavailable
                                                userInfo:@{
-                                                 NSLocalizedDescriptionKey :
-                                                     @"Failed to create call or channel."
-                                               }]];
+                                                          NSLocalizedDescriptionKey :
+                                                            @"Failed to create call or channel."
+                                                          }]];
     return;
   }
 
+  @synchronized (self) {
+    _wrappedCall = wrappedCall;
+  }
+
   [self sendHeaders];
   [self invokeCall];