JBIDE-16904 do not store when add already does store

.add already store the values.

.store result in calling .store(null) which gives NPE when concatenating.

for me looks like StringsPreference* has broken/inconsistent API.

Removing this does though reveal that the field will store *ANY* value - even values that does not validate nor have been used. filling up the list with "noise". I'll open separate issue for that.