private static void checkState(final DBExtensionType type, @Nullable final String key, final boolean nullable) {
            if (type == DBEXTENSION_INVALID) {
                throw new IllegalStateException("DBExtension: type must be set to valid type");
            }
            if (!StringUtils.isNotBlank(key) && !(nullable && null == key)) {
                throw new IllegalStateException("DBExtension: key value must be set");
            }
        }
removes all elements with this key from database
main/src/main/java/cgeo/geocaching/storage/DataStore.java