MapChunk.Fixtures can contain non-existent fixtures

[ElectroJr]

Reported on discord

Can cause the client to freeze due to exceptions while rendering. It'd be easy to turn the dictionary access into a try-get, but that's just hiding some other (potentially also severe) bug under the rug.

System.Collections.Generic.KeyNotFoundException: The given key 'grid_chunk-32'01-0'01' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Robust.Shared.Map.MapManager.IsIntersecting(ChunkEnumerator enumerator, IPhysShape shape, Transform shapeTransform, Entity`1 grid) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Map/MapManager.Queries.cs:line 29
   at Robust.Shared.Map.MapManager.<>c__65`1.<FindGridsIntersecting>b__65_0(GridQueryState`1& state, Proxy proxy) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Map/MapManager.Queries.cs:line 174
   at Robust.Shared.Physics.B2DynamicTree`1.Query[TState](TState& state, QueryCallback`1 callback, Box2& aabb) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Physics/B2DynamicTree.cs:line 894
   at Robust.Shared.Map.MapManager.FindGridsIntersecting[TState](EntityUid mapEnt, IPhysShape shape, Box2 worldAABB, Transform transform, TState& state, GridCallback`1 callback, Boolean approx, Boolean includeMap) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Map/MapManager.Queries.cs:line 139
   at Robust.Shared.Map.MapManager.FindGridsIntersecting[TState](EntityUid mapEnt, Box2 worldAABB, TState& state, GridCallback`1 callback, Boolean approx, Boolean includeMap) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Map/MapManager.Queries.cs:line 227
   at Robust.Shared.Map.MapManager.FindGridsIntersecting[TState](MapId mapId, Box2 worldAABB, TState& state, GridCallback`1 callback, Boolean approx, Boolean includeMap) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/Map/MapManager.Queries.cs:line 67
   at Robust.Shared.ComponentTrees.ComponentTreeSystem`2.GetIntersectingTrees(MapId mapId, Box2 worldAABB) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/ComponentTrees/ComponentTreeSystem.cs:line 272
   at Robust.Shared.ComponentTrees.ComponentTreeSystem`2.QueryAabb(MapId mapId, Box2Rotated worldBounds, Boolean approx) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/ComponentTrees/ComponentTreeSystem.cs:line 305
   at Content.Client.Gameplay.GameplayStateBase.GetClickableEntities(MapCoordinates coordinates, IEye eye) in /home/runner/work/space-station-14/space-station-14/Content.Client/Gameplay/GameplayStateBase.cs:line 140
   at Content.Client.Sprite.SpriteFadeSystem.FrameUpdate(Single frameTime) in /home/runner/work/space-station-14/space-station-14/Content.Client/Sprite/SpriteFadeSystem.cs:line 56
   at Robust.Shared.GameObjects.EntitySystemManager.FrameUpdate(Single frameTime) in /home/runner/work/RobustToolbox/RobustToolbox/Robust.Shared/GameObjects/EntitySystemManager.cs:line 333
[ERRO] ui: Caught exception while trying to draw a UI element: MainWindowRoot (WindowRoot)

Probably either revealed by, or caused by #5348?