Rule with lastUpdate fails with null?

I have a rule where I’m looking on the time of the last update. I always get the following error:

2018-03-02 18:41:19.248 [ERROR] [ntime.internal.engine.RuleEngineImpl] - Rule 'BLNet Raw auswerten': cannot invoke method public abstract long org.joda.time.ReadableInstant.getMillis() on null

I toggled the value of the item several times to be sure that there is a last value, but the error still occurs.

The code where it happens is in the first code line:

// nach ca. 60 Minuten, danach alle 24 Stunden Nachricht versenden
val LastReset = swi_Heiz_ResetBLNet.lastUpdate.getMillis
val Jetzt = now.millis
val Number Diff = Minutes::minutesBetween(new DateTime(LastReset), new DateTime(Jetzt)).getMinutes()
if (Diff.intValue % (24 * 60) == 60)
	sendTelegram("HausWartung", "BLnet benötigt Reset!")

The Item is defined as

Switch	swi_Heiz_ResetBLNet		"BLnet: Reset nötig"		<none>		(gRestoreOnStartup)

The group gRestoreOnStartup defines persistence via mapdb. This works for other items without problems.

Can you help me?

Is mapdb the default persistence service? otherwise you will have to use

val LastReset = swi_Heiz_ResetBLNet.lastUpdate("mapdb").millis


EDIT: Please be aware that, according to good old Designer, it’s .millis, not .getMillis

MapDB is defined as standard, but I also tried with the version you proposed. it does not make any difference.
Also if I use .millis or .getMillis.