Test get_idx_by_id and fix panic when not found #20

All 170 tests pass in 1s

170 tests +1 170 ✔️ +1 1s ⏱️ -1s
    2 suites ±0     0 💤 ±0
    1 files ±0     0 ❌ ±0

Results for commit a8db4b0. ± Comparison against earlier commit 42e6983.