fix random crash on interface shutdown

I believe this will fix some of the random occurrences of this crash...

```
Thread 4 Crashed:: QThread
0   QtCore                        	0x000000010ad1c850 QMutexPrivate::allocate() + 208
1   QtCore                        	0x000000010ad1c48b QBasicMutex::lockInternal(int) + 155
2   QtCore                        	0x000000010ad1c3d7 QMutex::lock() + 119
3   QtCore                        	0x000000010ad1f403 QThread::exit(int) + 35
4   interface                     	0x000000010a63b91b GenericThread::threadRoutine() + 139 (GenericThread.cpp:72)
5   interface                     	0x000000010a651c8b GenericThread::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) + 107 (moc_GenericThread.cpp:75)
6   QtCore                        	0x000000010af0e591 QMetaObject::activate(QObject*, int, int, void**) + 2593
7   QtCore                        	0x000000010ad22e47 QThreadPrivate::start(void*) + 423
8   libsystem_c.dylib             	0x00007fff89ded7a2 _pthread_start + 327
9   libsystem_c.dylib             	0x00007fff89dda1e1 thread_start + 13
```