Panic when removing rigid body in a multi body

Scenario: A chain of rigid bodies attached to a fixed body using multi body joint. Another rigid body placed on top of the fixed body (Won't crash if this body is not added). Crash happens when removing the last body connected to the fixed body. Backtrace: ```bash thread 'main' (650592) panicked at crates/rapier3d/../../src/dynamics/solver/contact_constraint/generic_contact_constraint.rs:63:35: No element at index stack backtrace: 0: __rustc::rust_begin_unwind 1: core::panicking::panic_fmt 2: core::option::expect_failed 3: rapier3d::dynamics::solver::contact_constraint::generic_contact_constraint::GenericContactConstraintBuilder::generate 4: rapier3d::dynamics::solver::contact_constraint::contact_constraints_set::ContactConstraintsSet::init 5: rapier3d::dynamics::solver::island_solver::IslandSolver::init_and_solve 6: rapier3d::pipeline::physics_pipeline::PhysicsPipeline::step 7: rapier_testbed3d::harness::Harness::step_with_graphics 8: rapier_testbed3d::testbed::update_testbed 9: <bevy_ecs::system::function_system::FunctionSystem<Marker,F> as bevy_ecs::system::system::System>::run_unsafe ``` Example for reproduction: newDINO/rapier@7d53edd