Initialize all the Invader instances in the prepareLevel function

KotlinInvadersView.kt

private fun prepareLevel() {
   // Here we will initialize the game objects
   // Build an army of invaders
   Invader.numberOfInvaders = 0
   numInvaders = 0
   for (column in 0..10) {
       for (row in 0..5) {
           invaders.add(Invader(context,
                   row,
                   column,
                   size.x,
                   size.y))

           numInvaders++
       }
   }
}