[Editor] .spm tree models are not being filtered by 'Search by Model' option.

Steps to Reproduce:
1. Open the attached or create a new project
2. Import 'Free SpeedTrees' package from Asset Store.
3. Filter Project view by model, by clicking the corresponding button in 'Search by type' dropdown list.

Actual Result:
.spm tree models are missing in the searched result.

Expected Result:
Available .spm tree models should be displayed in the searched result.

Environment:
Windows and Mac

Occurring with:
2018.3.0b2, 2018.3.0b1, 2018.3.0a6

Note:
Prefab Editor was introduced in 2018.3.0a6

Working fine with:
2018.3.0a5, 2018.2.0f2