More specifically the following sequence of events cause the crash:

1. A secondary is restarted with --shardsvr
2. The secondary applies a create DDL command oplog entry for collection X
3. Collection metadata for X is cleared 
4. The secondary becomes primary and runs an insert on X
5. The insert attempts to get the collection metadata for X, which was cleared, and so it attempts to throw a StaleConfig.
6. Throwing StaleConfig causes the server to crash because there is no ShardId to create the StaleConfig error as the rs hasn't been added as a shard yet.