How to reproduce:
1. Open the attached project

Actual result: Unity crashes.

Reproducible with: 2019.1.0a6, 2018.3.0b7, 2018.3.0a1,
Regression introduced in: 2018.3.0a1.
Not Reproducible with: 2018.2.14f1, 2018.1.9f2.

Notes:
In Versions where Unity doesn't crash, an error is thrown when re-importing the .spm files:
"SpeedTree file does not contain run-time data. Be sure to re-save it using the Unity-specific version of the SpeedTree Modeler" (May be related to a crash in later versions)

Unity freezes when importing the files and crashes when trying to exit (crash at mono_arch_unwind_frame).

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fixed in: 2019.1.0a9, 2018.3.0b10