Fixes a crash due to missing unique map init when creating new instances

Previously the following line would crash if this was the first interaction with the model, because `g_instancesReadLock` was not initialized.

    dispatch_semaphore_wait(g_instancesReadLock, DISPATCH_TIME_FOREVER);

The following lines would also not work without this change:

    if (! classCache) classCache = g_instances[(id) self.class] = [NSMapTable strongToWeakObjectsMapTable];