How to reproduce:
1. Open the attached "1260134 repro" project
2. Open the Addressables Groups window from Window > Asset Management > Addressables > Groups
3. Make a new build by clicking Build > New Build > Default Build Script in the Addressables Groups window

Expected result: a warning stating that the unrecognized Assets will be stripped is thrown
Actual result: the build fails

Reproducible with: 1.11.2 (2018.4.24f1, 2019.4.2f1, 2020.1.0b14, 2020.2.0a16)
Not reproducible with: 1.15 (2018.4.24f1), 1.8.4 (2020.1.0b14), 1.9.2 (2020.2.0a16), 1.10.0 (2018.4.24f1, 2019.4.2f1, 2020.1.0b14, 2020.2.0a16)

Notes:
-The same issue reproduces with other types of unrecognized Assets, for example, .abc and .usdc files
-In package versions below 1.11.2 the following warning is thrown:
Type DefaultAsset is in editor assembly UnityEditor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null. Asset location with internal id Assets/3D Models/untitled.blend1 will be stripped.
-When the build fails the following errors are thrown:
Build Task GenerateBundleCommands failed with exception:
Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

SBP ErrorException
UnityEditor.GenericMenu:CatchMenu(Object, String\[], Int32)