Reproduction steps:
1. Download "BundleVariant.zip" project and open in Unity
2. Press Repro -> Build Bundles (Manual)
3. Enter Play mode

Expected: no errors appear in the console and untagged_red and untagged_green cubes in Game view are red and green respectively
Actual: both cubes are pink and the error appears in the console

Reproduced on: 5.6.5p2; 2017.1.3p1; 2017.2.2p1; 2017.3.1p3; 2018.1.0b10; 2018.2.0a3

Note: error message - Variant folder path cannot be empty.
UnityEditor.BuildPipeline:BuildAssetBundles(String, AssetBundleBuild[], BuildAssetBundleOptions, BuildTarget)
Editor.BuildMenu:BuildBundlesManual() (at Assets/Editor/BuildMenu.cs:89)

Fixed on: 2018.3.0a8