Fix the safe level lookup in a mobile game

An AI wrote Kotlin game code that looked up a level's config with levelConfigs[id]!! and its music with soundtracks[config.themeId]!!. It worked for the campaign levels, then crashed with a NullPointerException on community and seasonal levels that the maps did not know. The fix looks both up safely and falls back instead of crashing.

An assistant wrote resolve_level_track(level_ids, level_themes, soundtrack_themes, requested_id). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• level_ids and level_themes: each known level's id and its theme, at the same index.
• soundtrack_themes: the themes that ship with a music track.
• requested_id: the level the player wants to start.
• Return missing_level, track:default or track:<theme>.

Rules the tests check:

• Find the first index where level_ids equals requested_id exactly, case included. If there is none, return missing_level.
• If level_themes has no entry at that index, return missing_level.
• If that level's theme is in soundtrack_themes, return track: followed by the theme. Otherwise return track:default.
• An empty theme is a theme like any other.