NPE when Json.toJson

Caused by: java.lang.NullPointerException
	at play.api.libs.json.LowPriorityWrites.$anonfun$traversableWrites$2(Writes.scala:307)
	at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at scala.collection.TraversableLike.map(TraversableLike.scala:234)
	at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
	at scala.collection.AbstractTraversable.map(Traversable.scala:104)
	at play.api.libs.json.LowPriorityWrites.$anonfun$traversableWrites$1(Writes.scala:307)
	at play.api.libs.json.Writes$$anon$6.writes(Writes.scala:142)
	at play.api.libs.json.PathWrites.$anonfun$nullable$2(JsConstraints.scala:184)
	at play.api.libs.json.OWrites$$anon$3.writes(Writes.scala:111)
	at play.api.libs.json.OWrites$MergedOWrites$.mergeIn(Writes.scala:75)
	at play.api.libs.json.OWrites$MergedOWrites$$anon$1.writeFields(Writes.scala:67)
	at play.api.libs.json.OWrites$MergedOWrites$$anon$1.writeFields(Writes.scala:63)
	at play.api.libs.json.OWrites$MergedOWrites$.mergeIn(Writes.scala:73)
	at play.api.libs.json.OWrites$MergedOWrites$$anon$1.writeFields(Writes.scala:66)
	at play.api.libs.json.OWrites$MergedOWrites$$anon$1.writeFields(Writes.scala:63)
	at play.api.libs.json.OWrites$OWritesFromFields.writes(Writes.scala:95)
	at play.api.libs.json.OWrites$OWritesFromFields.writes$(Writes.scala:93)
	at play.api.libs.json.OWrites$MergedOWrites$$anon$1.writes(Writes.scala:63)
	at play.api.libs.json.OWrites$$anon$2.$anonfun$contramap$1(Writes.scala:106)
	at play.api.libs.json.OWrites$$anon$3.writes(Writes.scala:111)
	at play.api.libs.json.OWrites$$anon$3.writes(Writes.scala:110)
	at play.api.libs.json.LowPriorityWrites.$anonfun$traversableWrites$2(Writes.scala:307)
	at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at scala.collection.TraversableLike.map(TraversableLike.scala:234)
	at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
	at scala.collection.AbstractTraversable.map(Traversable.scala:104)
	at play.api.libs.json.LowPriorityWrites.$anonfun$traversableWrites$1(Writes.scala:307)
	at play.api.libs.json.Writes$$anon$6.writes(Writes.scala:142)
	at play.api.libs.json.Json$.toJson(Json.scala:186)
	at controllers.FacadesController.$anonfun$queryTypes$2(FacadesController.scala:28)

Start at controllers/FacadesController.scala:28 and trace the value passed to Json.toJson into play.api.libs.json.LowPriorityWrites.traversableWrites in Writes.scala:307. Compare the nullable path in JsConstraints.scala:184 and Json.scala:186, then reproduce the NPE with a focused case; done means the cause is confirmed and covered by a regression test.