How to write an openHAB rule which increments a number?

I want to write a Rule in openHAB2, which increments a counter of all group items. The items:

Group counters
Number cnt1 (counters)
Number cnt2 (counters)

My try of a rule:

rule "Increase value .1 per minute"
when 
    Time cron "0 * * * * ?" or
    System started
then
    // Initialize. Necessary?
    counters?.members.forEach(counter|
        postUpdate(counter, 0.0)
    )
    counters?.members.forEach(counter|
        postUpdate(counter, 0.1 + counter.state)
    }
end

But that does not work. Exception: Error during the execution of startup rule 'Increase value .1 per minute': Could not invoke method: org.eclipse.xtext.xbase.lib.DoubleExtensions.operator_plus(double,byte) on instance: null

I tried to explore the type of counter.state, and with logInfo(counter.state.class) it correctly logs ...DecimalType.

it seems the implicit type casting doesn't seem to work. If you'd change the rule to something like:

rule "Increase value .1 per minute"
when 
    Time cron "0 * * * * ?" or
    System started
then
    // Initialize. Necessary?
    counters?.members.forEach(counter|
        if (counter.state == null) {
            postUpdate(counter, 0.0)
        }
    )
    counters?.members.forEach(counter|
        postUpdate(counter, 0.1 + (counter.state as DecimalType))
    }
end

it should work. Hope this helps,

Thomas E.-E.