Add this big chunk of code which handles collision detetion at the very end (but inside of) the update function.

KotlinInvadersView.kt

// Has the player's playerBullet 
// hit the top of the screen
if (playerBullet.position.bottom < 0) {
	playerBullet.isActive =false
}

// Has an invaders playerBullet 
// hit the bottom of the screen
for (bullet in invadersBullets) {
	if (bullet.position.top > size.y) {
		bullet.isActive = false
	}
}

// Has the player's playerBullet hit an invader
if (playerBullet.isActive) {
	for (invader in invaders) {
		if (invader.isVisible) {
			if (RectF.intersects(playerBullet.position, 
				invader.position)) {
					invader.isVisible = false

				soundPlayer.playSound(
					SoundPlayer.invaderExplodeID)
					
				playerBullet.isActive = false
				Invader.numberOfInvaders --
				score += 10
				if(score > highScore){
					highScore = score
				}

				// Has the player cleared the level
				//if (score == numInvaders * 10 * waves) {
				if (Invader.numberOfInvaders == 0) {
					paused = true
					lives ++
					invaders.clear()
					bricks.clear()
					invadersBullets.clear()
					prepareLevel()
					waves ++
					break
				}

				// Don't check any more invaders
				break
			}
		}
	}
}

// Has an alien playerBullet hit a shelter brick
for (bullet in invadersBullets) {
	if (bullet.isActive) {
		for (brick in bricks) {
			if (brick.isVisible) {
				if (RectF.intersects(bullet.position, brick.position)) {
					// A collision has occurred
					bullet.isActive = false
					brick.isVisible = false
					soundPlayer.playSound(SoundPlayer.damageShelterID)
				}
			}
		}
	}
}

// Has a player playerBullet hit a shelter brick
if (playerBullet.isActive) {
	for (brick in bricks) {
		if (brick.isVisible) {
			if (RectF.intersects(playerBullet.position, brick.position)) {
				// A collision has occurred
				playerBullet.isActive = false
				brick.isVisible = false
				soundPlayer.playSound(SoundPlayer.damageShelterID)
			}
		}
	}
}

// Has an invader playerBullet hit the player ship
for (bullet in invadersBullets) {
	if (bullet.isActive) {
		if (RectF.intersects(playerShip.position, bullet.position)) {
			bullet.isActive = false
			lives --
			soundPlayer.playSound(SoundPlayer.playerExplodeID)

			// Is it game over?
			if (lives == 0) {
				lost = true
				break
			}
		}
	}
}

if (lost) {
	paused = true
	lives = 3
	score = 0
	waves = 1
	invaders.clear()
	bricks.clear()
	invadersBullets.clear()
	prepareLevel()
}