Protect all SingleResultCallback observables from NPE

SCALA-555

Author: rozza

driver/src/it/scala/org/mongodb/scala/SmokeTestISpec.scala

@@ -49,6 +49,9 @@ class SmokeTestISpec extends RequiresMongoDBISpec with FuturesSpec {
  info("find first should return null if no documents")
  collection.find().first().futureValue shouldBe null // scalastyle:ignore
 
+ info("find should return an empty list")
+ collection.find().futureValue shouldBe empty
+
  info("Insert a document")
  collection.insertOne(document).futureValue should equal(Completed())
 

driver/src/main/scala/org/mongodb/scala/ObservableImplicits.scala

@@ -442,7 +442,10 @@ trait ObservableImplicits {
  subscription.request(1)
  }
 
- override def onError(throwable: Throwable): Unit = completeWith("onError", () => observer.onError(throwable))
+ override def onError(throwable: Throwable): Unit = throwable match {
+ case npe: NullPointerException => onComplete()
+ case _ => completeWith("onError", () => observer.onError(throwable))
+ }
 
  override def onComplete(): Unit = {
  completeWith("onComplete", { () =>