Single Cell Datasets: crashes on latest master

[ajdapretnar]

Describe the bug After https://github.com/biolab/orange3/pull/5279, Single Cell Datasets crashes.

To Reproduce Steps to reproduce the behavior:

1. Be on the latest Orange master.
2. Open Single Cell Datasets.

Expected behavior No crash.

Desktop (please complete the following information):

• OS: macOS
• Python Version: 3.8
• Orange Version: 3.28.dev
• Add-on Version: 1.2.0 (which ever is the latest)

Additional context

-------------------------- AttributeError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/ajda/orange/orange3/Orange/widgets/data/owdatasets.py", line 386, in __set_index
    scw(self.Header.variables, 20 + width("1000000"))
AttributeError: '_header_index' object has no attribute 'variables'
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