Good catch. It looks like it wasn't initialised. I have fixed this now in an additional commit.

It was supposed to get initialised dependent on slim or non slim mode. In non-slim mode, all tag strings are stored in memory. However, instead of storing highway=residential a gazillion times in memory, simple store a pointer to a generic string de-duplicating things and saving memory.

In slim mode tags get written out to the db directly and discarded from memory. So there is no need to go through the extra effort of de-duplicating the strings. Particularly when using threading, the de-duplication migh cost significant amount of overhead, as it results in lock contention on the de-duplication search tree.