See Features for Rules that work with Groups for details on Member of.
And I’ll agree with hr_2. You only get line numbers for syntax errors. Trying to use an object that is null or in a way that is not valid (e.g trying to parse a string into a number that cannot be parsed) will result in errors like the above.
What I don’t understand is why the rule executes six times when using the Member of trigger. Are all six updated at the same time?
Unless I misunderstand how it works, you shouldn’t have to loop through all the members for each trigger of the rule. The point of Member of is that it will cause triggeringItem to point to the member that was updated. So drop the loop, set aPrice to triggeringItem, drop the counter and you are good to go. The rule will be called once for each member of the group as they are individually updated.
I suspect what is happening is the first few times the rule triggers not all of the members have been populated so you are getting null exceptions. The last two work because by the time the last one runs all of the members have a usable value.