Bug: Deleting the only series in an Insight causes a server error and breaks rendering

Bug Description

It’s possible to delete the only remaining data series in an Insight (e.g. a single “count of events” series). After doing so, the Insight no longer renders and instead shows a server error.

This leaves the Insight in an invalid state. The UI should either prevent deleting the final series or render a safe empty state prompting the user to add data.

Observed error:
TypeError: Cannot read properties of undefined (reading 'math_group_type_index')

How to reproduce

1. Create a new Insight (e.g. Trends)
2. Add a single series (e.g. “Count of events”)
3. Save the Insight
4. Edit the Insight
5. Delete the only remaining series
6. The Insight fails to render and shows a server error

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/46625 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/49589)

Debug info

Kind: bug

Target area: analytics

Report event: http://go/ticketByUUID/3dfbf7b2-661c-4e26-ba09-b39204400645

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019ba1fe-e34e-79c2-841d-51930a7fc12a?t=105

Exceptions: https://us.posthog.com/project/2/error_tracking?filterGroup=%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22key%22%3A%22%24session_id%22%2C%22value%22%3A%5B%22019ba1fe-e34e-79c2-841d-51930a7fc12a%22%5D%2C%22operator%22%3A%22exact%22%2C%22type%22%3A%22event%22%7D%2C%7B%22type%22%3A%22hogql%22%2C%22key%22%3A%22uuid%20%3D%20'019ba1ff-cced-7ba4-b43d-cdaae8483b0b'%22%2C%22value%22%3Anull%7D%5D%7D%5D%7D

Location: https://eu.posthog.com/project/95192/insights/ufBqU3z3/edit

Persons-on-events mode for project: person_id_override_properties_on_events

Start by reproducing the deletion flow in the Insight editor described in steps 1–6, then trace the rendering path that fails on math_group_type_index. Done means deleting the final series no longer produces a server error: the UI either prevents the deletion or renders an empty state that prompts the user to add data.