How to reproduce:
1. Create a new project
2. Quit the Editor

Expected results: Editor quits successfully
Actual results: Editor quits, but the logs indicate that there was a fatal signal

Reproducible with: 2020.1.0a8
Not reproducible with: 2017.4.32f1, 2018.4.10f1, 2019.2.8f1, 2019.3.0b5, 2020.1.0a6
Regression from 2020.1.0a6 to 2020.1.0a7

First 5 stack trace frames:
#0 0x007f67898bb960 in __pthread_mutex_lock
#1 0x0055b7c1bf41e8 in FindAssetsWithImporterTypeV2(Unity::Type const*, std::set<UnityGUID, std::less<UnityGUID>, std::allocator<UnityGUID> >&, AssetDatabase::ImportSyncMode)
#2 0x0055b7c1bf48c2 in FindAssetImportersV2(Unity::Type const*, dynamic_array<AssetImporter*, 0ul>&, AssetDatabase::ImportSyncMode)
#3 0x0055b7c0e4b959 in PluginManager::RefreshPlugins(bool)
#4 0x0055b7c0e4c864 in PluginManager::GetPluginPath(core::basic_string<char, core::StringStorageDefault<char> > const&)

Notes:
- There is a very brief flash of a popup window when exiting which might indicate that 'Report a Bug' screen is crashing, since the Bug Reporter doesn't show up