Steps to reproduce:
1. Create a new project
2. In the project, window create a new brush

Results: Editor crashes or freezes

Reproduced with: 2019.1.0a3, 2018.3.0b7
Not reproducible with: 2019.1.0a2

========== OUTPUTTING STACK TRACE ==================

0x00000001405A26E3 (Unity) EventManager::InvokeEventCommon
0x00000001405A60CA (Unity) delete_object_internal_step1
0x00000001405A1343 (Unity) DestroySingleObject
0x0000000141E8D568 (Unity) PostprocessAssetsUpdatePreviews
0x0000000141E9324F (Unity) Postprocess
0x0000000141EB3A24 (Unity) AssetInterface::ApplyDefaultPostprocess
0x0000000141EC5316 (Unity) AssetInterface::ProcessAssetsImplementation
0x0000000141ECECC4 (Unity) AssetInterface::StopAssetEditing
0x0000000141E78099 (Unity) AssetDatabase::CreateSerializedAsset
0x0000000141E778DE (Unity) AssetDatabase::CreateSerializedAsset
0x00000001414071EC (Unity) AssetDatabase_CUSTOM_CreateAsset
<...>
========== END OF STACKTRACE ===========