scala> new V(null).toString
java.lang.NullPointerException: Cannot invoke "String.hashCode()" because "$this" is null
  at V$.hashCode$extension(<console>:1)
  at V.hashCode(<console>:1)
  at java.base/java.lang.Object.toString(Object.java:269)

Welcome to Scala 3.8.0 (25, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> class M(val s: String) extends AnyVal
[[syntax trees at end of                     typer]] // rs$line$2
package <empty> {
  final lazy module val rs$line$2: rs$line$2 = new rs$line$2()
  final module class rs$line$2() extends Object() { this: rs$line$2.type =>
    final class M(s: String) extends AnyVal() {
      val s: String
    }
    final lazy module val M: M = new M()
    final module class M() extends AnyRef() { this: M.type =>}
  }
}

// defined class M

scala> M(null)
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by scala.runtime.LazyVals$
WARNING: Please consider reporting this to the maintainers of class scala.runtime.LazyVals$
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
[[syntax trees at end of                     typer]] // rs$line$3
package <empty> {
  final lazy module val rs$line$3: rs$line$3 = new rs$line$3()
  final module class rs$line$3() extends Object() { this: rs$line$3.type =>
    val res0: M = new M(null)
  }
}

java.lang.NullPointerException: Cannot invoke "String.hashCode()" because "$this" is null
  at rs$line$2$M$.hashCode$extension(rs$line$2:1)
  at rs$line$2$M.hashCode(rs$line$2:1)
  at java.base/java.lang.Object.toString(Object.java:269)