Crash while scanning for plugins in the background

I have a VST that hosts other VSTs within it. When my VST is loaded, it starts by scanning the usual locations for AU plugins. I do this by spawning a few std::threads and passing them a PluginDirectoryScanner. Each thread just calls PluginDirectoryScanner::scanNextFile() until it returns false.

It works pretty much exactly like the plugin host demo.

However, after scanning a few plugins, it eventually crashes with a SIGABRT error. In the terminal, I see the message, "libc++abi.dylib: Pure virtual function called!"

Here is the stack trace (it's totally unhelpful):

Juce Message Thread (1)Queue : (serial)
#0    0x00007fff98e79002 in __pthread_kill ()
#1    0x000000010117c44e in pthread_kill ()
#2    0x00007fff9160e6e7 in abort ()
#3    0x00007fff87b7bf81 in abort_message ()
#4    0x00007fff87ba04dc in __cxa_pure_virtual ()
#6    0x00007fff87c9a853 in __CFRunLoopDoTimer ()
#7    0x00007fff87d18e6a in __CFRunLoopDoTimers ()
#8    0x00007fff87c55cd1 in __CFRunLoopRun ()
#9    0x00007fff87c55338 in CFRunLoopRunSpecific ()
#10    0x00007fff88547935 in RunCurrentEventLoopInMode ()
#11    0x00007fff8854776f in ReceiveNextEventCommon ()
#12    0x00007fff885475af in _BlockUntilNextEventMatchingListInModeWithFilter ()
#13    0x00007fff866c20ee in _DPSNextEvent ()
#14    0x00007fff86a8e943 in -[NSApplication _nextEventMatchingEventMask:untilDate:inMode:dequeue:] ()
#15    0x00007fff866b7fc8 in -[NSApplication run] ()
#16    0x00000001002758b3 in juce::MessageManager::runDispatchLoop() at /Users/kevin/Projects/JUCE/modules/juce_events/native/
#17    0x000000010027579a in juce::JUCEApplicationBase::main() at /Users/kevin/Projects/JUCE/modules/juce_events/messages/juce_ApplicationBase.cpp:240
#18    0x00000001002755ac in juce::JUCEApplicationBase::main(int, char const**) at /Users/kevin/Projects/JUCE/modules/juce_events/messages/juce_ApplicationBase.cpp:218
#19    0x0000000100011533 in main at /Users/kevin/Projects/JUCE/examples/audio plugin host/Source/HostStartup.cpp:119
#20    0x00007fff873de5ad in start ()

The weird thing is that if I block the main thread while the scan is in progress, it doesn't crash, and the scan finishes successfully.

I've been banging my head on the wall on this one for a while now. Hoping you guys might have some ideas. Thanks in advance!