[BUG] Create rollup jobs shows empty error toast when data source switches to incompatible

[xeniatup]

What is the bug?
When user switches from a compatible data source to incompatible on "Create rollup job" page, the page displays empty error toast.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Go to future playground
2. Click on 'Rollup jobs', then create rollup job
3. on the first create page, select data source called aos211
4. View the available indexes in the source index field

5. switch data source to "incompatible" 6. see the empty error toast

What is the expected behavior?
The incompatible data sources should be hidden from the data source picker.
In the interim we can lock the previously selected data source to "read only" on the "Create" page.

[dblock]

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