Fix for NPE in NotificationHelper

[vbuberen]

📄 Context

Fix, which should protect from issues, like #168 in NotificationHelper. Checks for transaction in transactionBuffer being null should protect from cases described in the mentioned issue.

📝 Changes

Add explicit check for null value in transactionBuffer.

[cortinico]

Not really a blocker, but is probably better to extract that transactionBuffer.valueAt(i) now?

[cortinico]

How about something like:

(transactionBuffer.size() - 1 downTo 0).forEach { i ->
                    val item = transactionBuffer.valueAt(i)
                    if (item != null && count < BUFFER_SIZE) {
                        if (count == 0) {
                            builder.setContentText(item.notificationText)
                        }
                        inboxStyle.addLine(item.notificationText)
                    }
                    count++
                }