How to reproduce:
1. Open the user-provided project
2. In Project window, select "cone_ground_smoke.fbx" asset
3. Holding "Ctrl" key, select "spiral mesh vertical.fbx" as well
4. Observe Console window

Expected result: No errors are thrown.
Actual result: NullReference exception is thrown.

Reproducible with - 2019.1.0a10, 2019.1.0a14, 2019.2.0a2
Not reproducible with - 2017.4.19f1, 2018.3.3f1, 2019.1.0a9

Error thrown:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ModelImporterMaterialEditor.BuildExternalObjectsCache () (at C:/buildslave/unity/build/Modules/AssetPipelineEditor/ImportSettings/ModelImporterMaterialEditor.cs:212)
UnityEditor.ModelImporterMaterialEditor.OnEnable () (at C:/buildslave/unity/build/Modules/AssetPipelineEditor/ImportSettings/ModelImporterMaterialEditor.cs:183)
UnityEditor.AssetImporterTabbedEditor.OnEnable () (at C:/buildslave/unity/build/Modules/AssetPipelineEditor/ImportSettings/AssetImporterTabbedEditor.cs:23)
UnityEditor.ModelImporterEditor.OnEnable () (at C:/buildslave/unity/build/Modules/AssetPipelineEditor/ImportSettings/ModelImporterEditor.cs:19)