NullPointerException when calling Client chooseConnection()

This is similar to TINKERPOP-2597 and reported fixed in gremlin driver 3.5.2. However, we are still getting intermittent null pointer exceptions, and our stack trace it a bit different:

java.lang.NullPointerException: null
at org.apache.tinkerpop.gremlin.driver.Client$SessionedClient.chooseConnection(Client.java:813)
at org.apache.tinkerpop.gremlin.driver.Client.submitAsync(Client.java:390)
at org.apache.tinkerpop.gremlin.driver.Client.submitAsync(Client.java:373)
at org.apache.tinkerpop.gremlin.driver.Client.submitAsync(Client.java:298)
at org.apache.tinkerpop.gremlin.driver.Client.submit(Client.java:250)

In addition we see this in our logs:

class=org.apache.tinkerpop.gremlin.driver.ConnectionPool
Signalled closing of connection pool on Host{..}

class=org.apache.tinkerpop.gremlin.driver.Client
Could not initialize client for Optional[..]

Seeing a lot of this too:

Timed-out (16000 MILLISECONDS) waiting for connection on Host{}