Example that generates Null Pointer Exception trying to find implicits

Implicits.scala

package example
import cats._, data._
import cats.implicits._

abstract class Checker {

  type Config 
  type Err 
  type Log 
  implicit val logMonoid: Monoid[Log]

  type ReaderConfig[A] = Kleisli[Id, Config, A]
  type WriterEC[A] = WriterT[ReaderConfig, Log, A]
  type Check[A] = EitherT[WriterEC, Err, A]
  
  def ok[A](x: A): Check[A] = EitherT.pure[WriterEC,Err,A](x)
  def run[A](c: Check[A], config: Config): (Log,Either[Err,A]) =  
      c.value.run.run(config)
 
  // I add this line in order to obtain the implicit Monad of WriterEC
  val monadWriterEC = implicitly[Monad[WriterEC]]
  
}

object MyCheck extends Checker {
  type Config = String
  type Err = String
  type Log = String
  implicit val logMonoid: Monoid[Log] = new Monoid[Log] {
    def combine(l1: Log, l2: Log) = l1 + "\n" + l2
    def empty = ""
  }
}

object Test {
 import MyCheck._
 val c1: Check[Int] = ok(3)

 // If I don't add this line, it doesn't find implicit for Monad WriterEC
 implicit val mWriterEC = monadWriterEC 

 val c2 : Check[Int] = c1.flatMap(v => ok(v + 1))
 
 val test = run(c2,"") // Generates Null pointer exception
 
}

It can be solved using a lazy val monadWriterEC = ...