Debug crash when searching for enzymes

1. Open data/samples/ABIF/A01.abi.
2. Find restriction sites.
3. Select All.
4. Click OK.
5. Open 23 more DNA sequences less than 10,000 in length.
   Expected: an "Auto-annotations update task" will be run for each.
6. Wait for the tasks to finish. It will take a very long time.
   Expected: tasks completed successfully.
   Current: assert is triggered in QList.cpp:170

   void **QListData::append(int n)
   {
    Q_ASSERT(!d->ref.isShared());

   Our code that caused this: AnnotationGroup.cpp:255. Assert occurs at the stage "Promoting task {Create annotations} to 'Running'".

Possibly related to crash reports 1191, 1192, 1219, 1220. Perhaps https://stackoverflow.com/questions/45969457/qt-shared-ptr-signal-with-qthreadpool can help.