This code draws all the bullets for the player and the invaders. Add the code after the code which draws the shelters/bricks, as shown.

KotlinInvadersView.kt

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// Draw the bricks if visible
for (brick in bricks) {
    if (brick.isVisible) {
        canvas.drawRect(brick.position, paint)
    }
}

// Draw the players playerBullet if active
if (playerBullet.isActive) {
    canvas.drawRect(playerBullet.position, paint)
}

// Draw the invaders bullets
for (bullet in invadersBullets) {
    if (bullet.isActive) {
        canvas.drawRect(bullet.position, paint)
    }
}

// Draw the score and remaining lives
...
...