[BUG] pulsar.(*producer).Send() panic when topic is deleted

#### Expected behavior

When topic is deleted, should return `Topic Not Found` error instead of panic.

#### Actual behavior


#### Steps to reproduce

1. create topic;
2. create producer and send messages in loop;
3. force delete topic while producer is connected;
4. wait for a period of time, you can see the panic;

#### System configuration
**Pulsar version**: 0.11.1

Start with pulsar.(*producer).Send() and reproduce the reported sequence: create a topic, send in a loop, force-delete the topic, and continue sending. Inspect the failure after deletion and use the Pulsar version noted in the issue as context. Done means Send() returns a "Topic Not Found" error instead of panicking.

Written by the indexing model from the issue text.