Kotlin 顶级属性的初始化

我遇到了一些关于kotlin顶级属性的问题，下面是示例代码:

// Situation 1

class TreeNode(var `val`: Int) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

val tree2 = TreeNode(2)
val tree1 = TreeNode(3).apply { left = tree2 } 

fun main() {
    println(tree1.left) // prints "TreeNode@..."
}

// Situation 2

class TreeNode(var `val`: Int) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

val tree1 = TreeNode(3).apply { left = tree2 } 
val tree2 = TreeNode(2)

fun main() {
    println(tree1.left) // prints "null"
}

// Situation 3

class TreeNode(var `val`: Int) {
    var left: TreeNode? = throw Exception("I am an exception")
    var right: TreeNode? = null
}

val tree1 = TreeNode(3).apply { left = tree2 } 
val tree2 = TreeNode(2)

fun main() {
    /* prints "Exception in thread "main" java.lang.ExceptionInInitializerError 
     * Caused by: java.lang.Exception: I am an exception ..."
     */
    println(tree1.left) 
}

看来

1. 从上到下初始化顶级属性；
2. 如果依赖属性没有初始化，它将被默认值替换；

我说得对吗？如果是，我想知道

1. 有没有官方文件说这种行为？
2. 如果依赖属性尚未初始化，为什么它宁愿用默认值替换它，而不是出现编译时错误或抛出运行时错误，即tree2未初始化？

谢谢