NotNullVar.kt

<!-- Copyright 2020 Google LLC.
SPDX-License-Identifier: Apache-2.0 -->

private class NotNullVar<T : Any>() : ReadWriteProperty<Any?, T> {
   private var value: T? = null

   public override fun getValue(thisRef: Any?, property: KProperty<*>): T {
       return value ?: throw IllegalStateException("Property ${property.name} should be initialized before get.")
   }

   public override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
       this.value = value
   }
}