[macOS] Crash on shutdown

Describe the bug
On shutdown, the plugin crashes in DeInit:
Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       UNKNOWN_0xD at 0x0000000000000000
Exception Codes:       0x000000000000000d, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Termination Reason:    Namespace SIGNAL, Code 11 Segmentation fault: 11
Terminating Process:   exc handler [2864]

VM Region Info: 0 is not in any region.  Bytes before following region: 4318646272
      REGION TYPE                    START - END         [ VSIZE] PRT/MAX SHRMOD  REGION DETAIL
      UNUSED SPACE AT START
--->  
      __TEXT                      101695000-101fb9000    [ 9360K] r-x/r-x SM=COW  ...MacOS/OpenCPN

Thread 0::  Dispatch queue: com.apple.main-thread
0   libsystem_pthread.dylib             0x7ff810ca8c81 _pthread_mutex_firstfit_lock_slow + 147
1   libwx_baseu-3.2.0.2.1.dylib            0x102b21ac4 wxMutex::Lock() + 52
2   libobjsearch_pi.dylib                  0x11543f398 objsearch_pi::DeInit() + 488
3   OpenCPN                                0x101bb698b PluginLoader::DeactivatePlugIn(PlugInContainer*) + 555
4   OpenCPN                                0x101bbbadd PluginLoader::DeactivateAllPlugIns() + 77
5   OpenCPN                                0x10188da1a MyFrame::OnCloseWindow(wxCloseEvent&) + 2698
6   libwx_baseu-3.2.0.2.1.dylib            0x102b308ff wxEventHashTable::HandleEvent(wxEvent&, wxEvtHandler*) + 415
7   libwx_baseu-3.2.0.2.1.dylib            0x102b3156a wxEvtHandler::ProcessEvent(wxEvent&) + 186
8   libwx_baseu-3.2.0.2.1.dylib            0x102b31651 wxEvtHandler::ProcessEventLocally(wxEvent&) + 145
9   libwx_baseu-3.2.0.2.1.dylib            0x102b31510 wxEvtHandler::ProcessEvent(wxEvent&) + 96
10  libwx_baseu-3.2.0.2.1.dylib            0x102b31a1c wxEvtHandler::SafelyProcessEvent(wxEvent&) + 12
11  libwx_osx_cocoau_core-3.2.0.2.1.dylib          0x10315e258 wxWindowBase::Close(bool) + 88
12  libwx_osx_cocoau_core-3.2.0.2.1.dylib          0x103045599 -[wxNonOwnedWindowController windowShouldClose:] + 41
nohal / objsearch_pi

[macOS] Crash on shutdown #20
