Steps to repro:
1. Create new project;
2. Go to PlayerSettings window (Edit -> Project Settings -> Player);
3. Switch scripting runtime version to .NET 4.6 by selecting it in corresponding dropdown list;
4. Save project;
5. Restart Editor;
6. Go to PlayerSettings window (Edit -> Project Settings -> Player);
7. Import attached package by dragging it to Project folder.

Expected result:
- API Updater launches;
- Package is imported without errors;

Actual result:
- API Updater doesn't launch;
- Empty uninformative error is thrown in the console (see attached screenshot).

The error is triggered by 'Map_tc.js' script, located in Assets -> Editor, that failed to upgrade.

Workaround is to restart Editor - in this case API Updater launches correctly.

Notes:
- Reproducible in 2017.1.0b1;
- Reproducible only for .NET 4.6 Scripting Backend; Not reproducible for .NET 3.5 and for Mono 2x Scripting Backends;
- Reproducible only when PlayerSettings window is active;
- Reproducible on both Windows 10 and OSX 10.12.3;
- Reproducible only for this particular .js file.