Repro steps:
- download the package
- try to import it
- notice it crashes before even starting the scene
-Cannot reproduce in 2018.3.0b4

========== OUTPUTTING STACK TRACE ==================

0x0000000140BCF248 (Unity) GetNullUnityEngineObjectReplacement
0x0000000140BCF62E (Unity) TransferPPtrToMonoObject
0x0000000140C26035 (Unity) Transfer_UnityEngineObject<YAMLRead,0>
0x0000000140BA295E (Unity) TransferScriptingObject<YAMLRead>
0x0000000140BA049A (Unity) SerializableManagedRefTransfer::TransferMonoInstance<YAMLRead>
0x0000000140BA321B (Unity) SerializableManagedRefTransfer::TransferTextBased<SerializableManagedRefBackupGenerator,YAMLRead>
0x0000000140BB6BEA (Unity) MonoBehaviour::VirtualRedirectTransfer
0x0000000140C601F4 (Unity) SerializedFile::ReadObject
0x0000000140C4468F (Unity) PersistentManager::ReadAndActivateObjectThreaded
0x0000000140C41C8C (Unity) PersistentManager::LoadFileCompletelyThreaded
0x0000000140F82C22 (Unity) PrefabImporter::GenerateAssetData
0x0000000141EC5941 (Unity) AssetDatabaseV1::ImportAsset
0x0000000141ED7078 (Unity) AssetDatabaseV1::UpdateAsset
0x0000000141ECB418 (Unity) AssetInterface::ProcessAssetsImplementation
0x0000000141ED5364 (Unity) AssetInterface::StopAssetEditing
0x0000000141ED0870 (Unity) AssetInterface::RefreshInternal
0x0000000141ECD77C (Unity) AssetInterface::Refresh
0x0000000141E88485 (Unity) AssetDatabase::Refresh
0x00000001412A4DF1 (Unity) ImportPackageAssets
0x00000001412AA3F2 (Unity) TickPackageImport
0x000000014131C5D7 (Unity) Application::TickTimer
0x000000014147700B (Unity) MainMessageLoop
0x0000000141478CA6 (Unity) WinMain
0x000000014244F6BA (Unity) __scrt_common_main_seh
0x00007FFC69F63034 (KERNEL32) BaseThreadInitThunk
0x00007FFC6A0B1461 (ntdll) RtlUserThreadStart

========== END OF STACKTRACE ===========