Although documented (https://mongodb.github.io/mongo-java-driver/3.5/javadoc/com/mongodb/async/client/FindIterable.html#filter-org.bson.conversions.Bson-) as @Nullable, passing a null filter to FindIterable#find causes a NullPointerException to be thrown:

at com.mongodb.internal.operation.Operations.createFindOperation(Operations.java:141)
at com.mongodb.internal.operation.Operations.find(Operations.java:129)
at com.mongodb.internal.operation.AsyncOperations.find(AsyncOperations.java:84)
at com.mongodb.async.client.FindIterableImpl.createFindOperation(FindIterableImpl.java:228)
at com.mongodb.async.client.FindIterableImpl.asAsyncReadOperation(FindIterableImpl.java:224)
at com.mongodb.async.client.MongoIterableImpl.batchCursor(MongoIterableImpl.java:161)
at com.mongodb.async.client.MongoIterableSubscription.requestInitialData(MongoIterableSubscription.java:46)
at com.mongodb.async.client.AbstractSubscription.tryRequestInitialData(AbstractSubscription.java:151)
at com.mongodb.async.client.AbstractSubscription.request(AbstractSubscription.java:84)