I wrote a small test function. It appears that Scala.js has no default protections against null values from the browser DOM.

  def printNodes(targetNode: dom.Node): Unit = {
    var c = targetNode.firstChild        
    while (c != null) {
      if (c.nodeValue != null) {
        println(c.nodeValue)
      }
      c = c.nextSibling
    }
  }

The return value from firstChild and nextSibling can be null.