Scala: a function to generate a random-length string with blank spaces

/**
 * Produces random-length strings like these:
 *    genRandomVariableLengthStringWithBlankSpaces(r, 3, 15, "abcdef")
 *    "bff bcf"
 *    "f cbfe cff"
 */
def genRandomVariableLengthStringWithBlankSpaces(
    r: Random,
    minLength: Int,
    maxLength: Int,
    charsToUse: String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
): String =
    val ab = scala.collection.mutable.ArrayBuffer[Char]()
    val strLength = randomIntBetween(minLength, maxLength)
    for i <- 1 to strLength do
        if i % 5 == 0 then
            ab.append(' ')
        else
            // ab.append(r.nextPrintableChar)
            val randomChar = charsToUse(Random.nextInt(charsToUse.length))
            ab.append(randomChar)
    val charSeq: scala.collection.mutable.Seq[Char] = Random.shuffle(ab)
    charSeq.mkString