PR #4490 breaks new slice creation

[Pek1s]

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• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

master (4135854)

Expected results

When creating new Slice through /slicemodelview/add, Superset should allow selecting the datasource and visualization type and then continue into slice editor.

Actual results

Submitting the form causes TypeError exception and user is unable to get into slice editor.

superset/views/core.py", line 1199, in datasource_info causes the exception because there isn't datasource defined in encoded JSON that is provided as query parameter.

Possible fix

React frontend is still conforming partially old API and the changes #4490 are not propagated fully into frontend.
Frontend should inject the form parameters into form_data-param

EDIT: #4538 fixes this problem. See the PR

Steps to reproduce

1. Go to: /slicemodelview/add
2. Select any datasource
3. Select any visualization type
4. Click Create new slice