How to reproduce:
1. Open the attached "1051844_repro.zip" project
2. Observe that it contains three prefabs forming a hierarchy:
- "Parent" prefab contains nested "Child" prefab
- "Child" prefab contains nested "Grandchild" prefab
- "Parent" prefab has an empty script attached to the "Grandchild"
3. Inside "Child" prefab, delete the nested "Grandchild" GameObject

Expected result: Editor does not crash.
Actual result: Editor crashes.

Reproducible with - 2018.2.0x-ImprovedPrefabs (ff19e2e229d2)

Note: There is another scenario to reproduce this issue. If the project had already crashed before, reimport "Parent" prefab and the project crashes again.

Stack trace:
(Unity) PrefabImporter::GenerateAssetData
(Unity) AssetDatabaseV1::ImportAsset
(Unity) AssetDatabaseV1::UpdateAsset
(Unity) AssetInterface::ProcessAssetsImplementation
(Unity) AssetInterface::StopAssetEditing
(Unity) AssetInterface::ImportAtPath
<...>

Fixed in 2018.3.0a6