Reproduction steps:
1. Open the attached project "ScriptedImporterNull - 2018.1.zip";
2. Open the test.unity scene;
3. Make sure that it is set in the Vox Object of the Test script of GameObject to 3x3x3 Voxel object;
4. Open VoxScriptedImporter.cs and change "false" on the first line to "true";
5. Observe the changes of GameObject.

Expected results: The reference to Vox object is still kept on the GameObject.
Actual result: GameObject loses its reference to Vox object.

Reproduced on: 2017.1.2p4, 2017.2.1p2, 2017.3.0p4, 2018.1.0f2, 2018.2.0a1.