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
WidgetInspectorServer: Fix dangling pointer
Remove too-strict assert

QPointer is not yet aware of the object having been destroyed.
There's slighly odd behavior with QPointer tracking QWidget instances.
In short: QObject::destroyed is emitted *before* QPointers are cleared
if the tracking object inherits from QWidget

Also see:
https://codereview.qt-project.org/#change,40091
Merge remote-tracking branch 'origin/2.0'