Ensure DispatchDataSourcesView handles missing pages properly.

Corresponds to Sentry issue 5045432287.

• Create a page
• Add a input element to it
• Preview the page in another tab
• Return to the editor, trash the page
• In the preview tab, type in the input element, which will trigger a POST /api/builder/page/{id}/dispatch-data-sources/ request. The backend will fail hard.