Why you should care

var a: Boolean?  // a is a nullable boolean

Before accessing the value, we should verify if the variable is null or not. This can be done with the classical check : if … else … For exemple, to check against true:

if ((a != null) && (a == true)) { ... } else { ... }

In Kotlin it is possible to use a compact syntax using the elvis operator. (a ?: b) is equivalent to (if (a !=null) a else b). So checking a nullable Boolean to true can be shortly done with the elvis operator like that:

if ( a ?: false ) { ... } else { .... }

Unfortunately, even this is shortest it is nevertheless worse and indigest for many persons because of the boolean inversion. So, prefer using the direct check to the value, without checking the nullity:

if ( a == true ) { ... } else { .... }