fix-rollbar (4783/465148540610): guard against null PopStateEvent.state in rep max calculator

[astashovai]

Summary

• Add optional chaining (?.) when accessing e.state.reps in the popstate event handler in RepMaxContent
• PopStateEvent.state can be null when navigating to a history entry created by normal navigation (not pushState/replaceState), causing a TypeError

Rollbar

https://app.rollbar.com/a/astashov/fix/item/liftosaur/4783/occurrence/465148540610

Decision

Fixed because this is a legitimate bug in our code on a public-facing page (rep max calculator). The error affects normal user flows — users hitting browser back/forward can trigger this crash.

Root Cause

The popstate event handler in RepMaxContent accesses e.state.reps without guarding against e.state being null. The PopStateEvent.state property is null for history entries not created via pushState/replaceState (e.g., the initial page load entry). When a user navigates back to such an entry, the handler crashes with TypeError: null is not an object (evaluating 'e.state.reps').

Test plan

• Navigate to any rep max calculator page (e.g. /ten-rep-max-calculator)
• Use browser back/forward buttons — should not crash
• Change reps input and verify URL updates correctly
• Use back button to navigate between rep max states — reps should restore from history state