Fix crash when removing a collider with no graph ID.

src/geometry/narrow_phase.rs

@@ -137,8 +137,9 @@ impl NarrowPhase {
         let mut i = 0;
 
         while let Some(collider) = colliders.removed_colliders.read_ith(&cursor, i) {
-            let graph_idx = self.graph_indices.get(collider.handle).unwrap();
-
+            // NOTE: if the collider does not have any graph indices currently, there is nothing
+            // to remove in the narrow-phase for this collider.
+            if let Some(graph_idx) = self.graph_indices.get(collider.handle) {
                 let proximity_graph_id = prox_id_remap
                     .get(&collider.handle)
                     .copied()
@@ -156,6 +157,7 @@ impl NarrowPhase {
                     &mut prox_id_remap,
                     &mut contact_id_remap,
                 );
+            }
 
             i += 1;
         }