Steps to reproduce:

1) Open Unity Hub and create a new project
2) Go to Assets >> Import Package >> Custom package
3) Select provided asset package (.unitypackage)
4) Observe the crash while importing process is in progress

Reproduced with:
2019.3.0a6, 2019.3.0a8

Not reproduced with:
2019.2.0b9, 2019.3.0a5

Devices under testing:
MacBook Pro Intel Core i7 2,8 GHz 16 GB ; Intel HD Graphics 630 1536 MB Radeon Pro 555 2 GB ; Mac OS X 10.14.5 18F132 Darwin 18.6.0
Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz; NVIDIA GeForce GTX 1050 Ti; Windows 10 (10.0.0) 64bit
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========== OUTPUTTING STACK TRACE ==================

0x00007FF614CE7990 (Unity) UnityYAML::Write::SerializeScalar
0x00007FF614CE779F (Unity) UnityYAML::Write::SerializeNode
0x00007FF614CE7667 (Unity) UnityYAML::Write::SerializeMappingValue
0x00007FF614CE744F (Unity) UnityYAML::Write::SerializeMapping
0x00007FF614CE7809 (Unity) UnityYAML::Write::SerializeNode
0x00007FF614CE7667 (Unity) UnityYAML::Write::SerializeMappingValue
0x00007FF614CE740F (Unity) UnityYAML::Write::SerializeMapping
0x00007FF614CE7809 (Unity) UnityYAML::Write::SerializeNode
0x00007FF614CE6D45 (Unity) UnityYAML::Write::Serialize
0x00007FF614CF548B (Unity) YAMLWrite::OutputToCachedWriter
0x00007FF614C9756A (Unity) SerializedFile::WriteObject
0x00007FF614C65269 (Unity) PersistentManager::WriteFile
0x00007FF614C66114 (Unity) PersistentManager::WriteFile
0x00007FF614C66620 (Unity) PersistentManager::WriteFile
0x00007FF614E73429 (Unity) WriteSerializedAsset
0x00007FF614E736A9 (Unity) WriteSerializedAssetsV2