Use poll instead remove to avoid exception

Author: Technoboy-

pulsar-broker/src/main/java/org/apache/pulsar/client/impl/RawReaderImpl.java

@@ -218,7 +218,10 @@ protected CompletableFuture<Void> failPendingReceive() {
         private void failPendingRawReceives() {
             List<CompletableFuture<RawMessage>> toError = new ArrayList<>();
             while (!pendingRawReceives.isEmpty()) {
-                toError.add(pendingRawReceives.remove());
+                final CompletableFuture<RawMessage> ret = pendingRawReceives.poll();
+                if (ret != null) {
+                    toError.add(ret);
+                }
             }
             toError.forEach((f) -> f.cancel(false));
         }