Get rid off overambitious assert

We might get null-pointers for the 'obj' var in some cases, this is due
the way the probe sends out model modification events. These are done in
the main-thread, and some of the object may have already been deleted in
another thread again, hence resulting in a null-pointer here.

This and other multi-threading-related issues should fix #31.

Related:
bc0aca9f601b7e525f1b25020226f3dbcacec8c3