A lesson in nullability with PagedList

In the last couple of days I published a new release of Worker to internal tester and with this release I saw a rise of crashes reported to Firebase caused by a NullPointerException.

The application is written in Kotlin (converted of the last year from Java), and I’ve been careful regarding the use of optional/nullable values. So, the rise of crashes was very confusing, and I’ve gone over the related code multiple times without finding any red flags.

The crashes occurred when the time report view¹ had been opened for one minute which caused me to believe that there were some issue with the “refresh active time interval”-feature.

In this view I populate a PagedListAdapter using a PagedList<TimeReportWeek> property from the view model. And to refresh active time interval I take the current list from the adapter and iterate though the list to find if any loaded time intervals is active, the code for this is as follows (located in the view model).

private fun findActivePosition(weeks: List<TimeReportWeek>): Int? {
    return weeks.filter(::containsActiveDay)
        .map(weeks::indexOf)
        .firstOrNull()
}

private fun containsActiveDay(week: TimeReportWeek): Boolean {
    return week.days.firstOrNull { it is TimeReportDay.Active } != null
}

Note that I’m declaring that the weeks argument is a List<TimeReportWeek>, but under the hood it’s actually a PagedList<TimeReportWeek>.

Since the release I’ve been trying to figure out what could cause the crashes, without being able to successfully reproduce the crash in a debug environment. And, this morning by chance I was tweaking the PagedList configuration² in order to see if I could optimize the load times and this caused the application to crash with a NullPointerException.

After attaching a debugger I noticed that the application crashed when we attempted to access the days property because week is null. And, for a few seconds I was confused as the list should not allow for null values by the type system. But then it hit me, using a PagedList when placeholders are enabled will produce null values for not yet loaded items.