Steps to repro:

1. Open Unity on OSX;
2. Open Asset Store (Window -> Asset Store);
3. Download any package;
4. Click 'Import' button.

Actual result:
Error
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PackageImportTreeView+PackageImportTreeViewDataSource.FetchData () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/PackageImportTreeView.cs:537)

is thrown (see attached screenshot).

Regression introduced in Unity 5.4.0b22.

Expected result:
Error should not be thrown after pressing "Import" button.

Notes:
- Reproducible in 5.4.0b22;
- Not reproducible in 5.4.0b21;
- Reproducible only on OSX;
- Not reproducible on Windows 10.