This was introduced in rBe5c0d4613a8943c712b57fb336997ecd78e6508e.

The issue is that the new fluid system does not use the pointcache
system for caching anymore, but still relies on pointcache for
other things. For example, it uses DEG_add_collision_relations
which internally creates relations with pointcache. Not sure if
there are other issues.

Ideally, this dependence should be resolved in one way or another
at some point, but that is out of scope for this fix.