How to reproduce:
1. Open "PackedIntVector_CRASH.zip" project
2. Open "Lava Demo 2.unity" scene
3. Enter Play mode
4. If Unity didn't crash, Exit Play mode
5. Repeat steps 3-4

Actual result: Unity crashes

Reproducible with: 2018.3.3f1, 2018.3.1f1, 2018.3.0b8.
Regression introduced in: 2018.3.0b8.
Not reproducible with: 2018.2.20f1, 2018.3.0b7, 2019.1.0a1, 2019.2.0a1.

Notes:
Sometimes the same crash happens on import.

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Fixed in 2019.1.0a1, 2018.3.10f1.

There is another issue that some meshes are missing for the lava (that was the cause of the crash)
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The issue was fixed on 2019.2.0a9 and 2019.1.0b8, 2018.3.10f1.