To reproduce:

1. Download attached "Repro_Hash128_Parse.zip" project and open in Unity
2. Open "SampleScene"
3. Select "Hash128" gameObject in Hierarchy
4. In Inspector press "Parse in main thread" button
5. Observe that everything works as Expected
6. Press "Parse in thread pool" button

Expected results: As before everything works as Expected - Hash is printed and no Errors are logged
Actual results: Inspector throws an Exception:

"UnityEngine.UnityException: Parse can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.
at (wrapper managed-to-native) UnityEngine.Hash128.Parse_Injected(string,UnityEngine.Hash128&)
at UnityEngine.Hash128.Parse (System.String hashString) [0x00000] in <15966b0e0b01470ab47c749d344008bb>:0
at Hash128ParserEditor.ParseHash () [0x00013] in /Users/jonas/Downloads/case_1185356-Repro_Hash128_Parse/Assets/Editor/Hash128ParserEditor.cs:38
UnityEngine.Debug:Log(Object)
Hash128ParserEditor:ParseHash() (at Assets/Editor/Hash128ParserEditor.cs:43)
Hash128ParserEditor:<OnInspectorGUI>b__2_0(Object) (at Assets/Editor/Hash128ParserEditor.cs:27)
System.Threading._ThreadPoolWaitCallback:PerformWaitCallback()"

Notes:
- This issue happens on both Windows and macOS
- Could not check on older Unity versions than 2018.3 because of old C# version

Reproduced on Unity 2018.4.10f1, 2019.2.7f2, 2019.3.0b5 and 2020.1.0a5