Fixes a crash that could happen when users tried to insert a pivot table from another spreadsheet sheet after reloading the page. This helps users continue working with spreadsheet reports reliably, even when some pivot data has not been loaded yet.

- Insert two pivots on a spreadsheet, one on the first sheet and one on the second
- Then reload the page and try to insert the second pivot (which is not loaded)
  in the first sheet.
=> boom

Task 2844664