spark null exception

exception.scala

import org.apache.log4j.{Level, Logger}
import org.apache.spark.SparkConf
import org.apache.spark.sql.functions._
import org.apache.spark.sql.{Dataset, SparkSession}

case class FooBar(city: String, postcode: String)

object Foo extends App {

  Logger.getLogger("org").setLevel(Level.WARN)

  val conf: SparkConf = new SparkConf()
    .setAppName("exception")
    .setMaster("local[*]")
    .set("spark.executor.memory", "2G")
    .set("spark.executor.cores", "4")
    .set("spark.default.parallelism", "4")
    .set("spark.driver.memory", "1G")
    .set("spark.serializer", "org.apache.spark.serializer.KryoSerializer")

  val spark: SparkSession = SparkSession
    .builder()
    .config(conf)
    //    .enableHiveSupport()
    .getOrCreate()

  import spark.implicits._

  implicit val e = org.apache.spark.sql.Encoders.DATE

  val dates = Seq(
    ("2016-01-01", "ABC"),
    ("2016-01-02", "ABC"),
    ("2016-01-03", "POL"),
    ("2016-01-04", "ABC"),
    ("2016-01-05", "POL"),
    ("2016-01-06", "ABC"),
    ("2016-01-08", "ABC"),
    ("2016-01-09", "POL"),
    ("2016-01-07", "POL"),
    ("2016-01-10", "ABC")
  ).toDF("dates", "ISO")
    .withColumn("dates", 'dates.cast("Date"))

  dates.show

  fit(dates)

  spark.stop

  def fit(df: Dataset[_]): Any = {
    import df.sparkSession.implicits._

    val dfFiltered = df
      .filter($"ISO" === "ABC")
    val someDate = dfFiltered
      .filter(_ != isnull('dates))
      .select(min('dates)).first.toSeq.head
    println(someDate)
  }
}