Based on the superclass:

//from page: 222
abstract class Element {
  def contents: Array[String]
  val height = contents.length
  val width = if (height == 0) 0 else contents(0).length
}

The book goes on to extend with another sub-class:

class UniformElement(
                    ch: Char,
                    override val width: Int,
                    override val height: Int
                    ) extends Element {
  private val line = ch.toString * width
  def contents = Array.fill(height)(line)
}

This causes a null-pointer exception in the Scala REPL.

I found that others also get trapped on this, possibly from older
versions of the book:
 -
https://stackoverflow.com/questions/42312232/npe-in-using-scala-array-fill
/42312929
 -
https://html.developreference.com/article/22372654/Scala+companion+object+
with+abstract+class

----
I added a footnote that clarifies which version of Element UniformElement
should extend, which is the one in Listing 10.2. If you do that, the NPE
doesn't happen.

Listing 10.6 - Invoking a superclass constructor.
it should revise to:

class LineElement(s: String) extends ArrayElement(Array(s)) {
  override val width = s.length
  override val height = 1
}


,not :

class LineElement(s: String) extends ArrayElement(Array(s)) {
override def width = s.length
override def height = 1
}

,otherwise it would report like this:

On line 2: error: stable, immutable value required to override:
       val width: Int (defined in class Element)
       	override def height = 1
                            ^
On line 3: error: stable, immutable value required to override:
       val height: Int (defined in class Element)

-----
The issue is from extending the wrong version of Element. I added
comments to clarify in 5ed. Tnx.