Add this after the call to invader.update and before the code which checks if an invader has bumped into the side of the screen

KotlinInvadersView.kt

...
...
invader.update(fps)

// Does he want to take a shot?
if (invader.takeAim(playerShip.position.left,
				playerShip.width,
				waves)) {

	// If so try and spawn a bullet
	if (invadersBullets[nextBullet].shoot(invader.position.left
					+ invader.width / 2,
					invader.position.top, playerBullet.down)) {

		// Shot fired
		// Prepare for the next shot
		nextBullet++

		// Loop back to the first one if we have reached the last
		if (nextBullet == maxInvaderBullets) {
			// This stops the firing of bullet
			// until one completes its journey
			// Because if bullet 0 is still active
			// shoot returns false.
			nextBullet = 0
		}
	}
}

// If that move caused them to bump
// the screen change bumped to true
...
...