Check for and handle null experiment instance in AMTLogin route

[disperse]

Check for and handle a null experiment or experiment instance here:

  public static Result amtLogin(Long experimentId, Long experimentInstanceId, String hitId, String assignmentId, String workerId) {
    Experiment experiment = Experiment.findById(experimentId);
    ExperimentInstance experimentInstance = ExperimentInstance.findById(experimentInstanceId);
    AMTHit amtHit = experimentInstance.getHit();

play.api.Application$$anon$1: Execution exception[[NullPointerException: null]]
at play.api.Application$class.handleError(Application.scala:293) ~[com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at play.api.DefaultApplication.handleError(Application.scala:399) [com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$2$$anonfun$applyOrElse$3.apply(PlayDefaultUpstreamHandler.scala:261) [com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$2$$anonfun$applyOrElse$3.apply(PlayDefaultUpstreamHandler.scala:261) [com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at scala.Option.map(Option.scala:145) [org.scala-lang.scala-library-2.10.2.jar:na]
at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$2.applyOrElse(PlayDefaultUpstreamHandler.scala:261) [com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
Caused by: java.lang.NullPointerException: null
at controllers.ClientLogin.amtLogin(ClientLogin.java:61) ~[breadboard.breadboard-v2.3.1.jar:2.2.0]
at Routes$$anonfun$routes$1$$anonfun$applyOrElse$50$$anonfun$apply$50.apply(routes_routing.scala:674) ~[breadboard.breadboard-v2.3.1.jar:na]
at Routes$$anonfun$routes$1$$anonfun$applyOrElse$50$$anonfun$apply$50.apply(routes_routing.scala:674) ~[breadboard.breadboard-v2.3.1.jar:na]
at play.core.Router$HandlerInvoker$$anon$7$$anon$2.invocation(Router.scala:183) ~[com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at play.core.Router$Routes$$anon$1.invocation(Router.scala:377) ~[com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]
at play.core.j.JavaAction$$anon$1.call(JavaAction.scala:56) ~[com.typesafe.play.play_2.10-2.2.0.jar:2.2.0]