Smart cast to ‘Type’ is impossible, because ‘variable’ is a mutable property that could have been changed by this time

Between execution of left != null and queue.add(left) another thread could have changed the value of left to null.

To work around this you have several options. Here are some:

1. Use a local variable with smart cast:

    val node = left
    if (node != null) {
        queue.add(node)
    }
   

2. Use a safe call such as one of the following:

    left?.let { node -> queue.add(node) }
    left?.let { queue.add(it) }
    left?.let(queue::add)
   

3. Use the Elvis operator with return to return early from the enclosing function:

    queue.add(left ?: return)
   

   Note that break and continue can be used similarly for checks within loops.