Panic in PushChildren command

## Bevy version - 0.9.1

## What you did

I have a particle emitter `Component` which spawns particles as its `Children`. It can also `despawn_recursive` itself when no longer needed. I'll try to localize a crash in the coming days as it is hard to pinpoint what exactly causes a crash.

## What went wrong

'Entity 457v0 does not exist' panic.

## Additional information

Crash happens here:

Full Backtrace

thread 'main' panicked at 'Entity 457v0 does not exist', /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/world/mod.rs:248:32

stack backtrace:

   0: rust_begin_unwind

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:575:5

   1: core::panicking::panic_fmt

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/panicking.rs:65:14

   2: bevy_ecs::world::World::entity_mut::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/world/mod.rs:248:32

   3: core::option::Option::unwrap_or_else

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/option.rs:828:21

   4: bevy_ecs::world::World::entity_mut

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/world/mod.rs:247:9

   5: ::write

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_hierarchy-0.9.1/src/child_builder.rs:168:26

   6: bevy_ecs::system::commands::command_queue::CommandQueue::push::write_command

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/system/commands/command_queue.rs:42:13

   7: bevy_ecs::system::commands::command_queue::CommandQueue::apply

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/system/commands/command_queue.rs:96:17

   8: ::apply

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/system/system_param.rs:595:9

   9: <(P0,P1,P2,P3) as bevy_ecs::system::system_param::SystemParamState>::apply

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/system/system_param.rs:1505:19

  10:  as bevy_ecs::system::system::System>::apply_buffers

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/system/function_system.rs:425:9

  11: ::run

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/schedule/stage.rs:838:29

  12: bevy_ecs::schedule::Schedule::run_once

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_ecs-0.9.1/src/schedule/mod.rs:370:13

  13: bevy_app::app::App::update

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_app-0.9.1/src/app.rs:152:9

  14: bevy_winit::winit_runner_with::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_winit-0.9.1/src/lib.rs:600:21

  15:  as winit::platform_impl::platform::app_state::EventHandler>::handle_nonuser_event::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/app_state.rs:106:17

  16: winit::platform_impl::platform::app_state::EventLoopHandler::with_callback

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/app_state.rs:80:13

  17:  as winit::platform_impl::platform::app_state::EventHandler>::handle_nonuser_event

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/app_state.rs:101:9

  18: winit::platform_impl::platform::app_state::Handler::handle_nonuser_event

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/app_state.rs:209:21

  19: winit::platform_impl::platform::app_state::AppState::cleared

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/app_state.rs:394:9

  20: winit::platform_impl::platform::observer::control_flow_end_handler::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/observer.rs:184:21

  21: winit::platform_impl::platform::observer::control_flow_handler::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/observer.rs:145:9

  22: std::panicking::try::do_call

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:483:40

  23: std::panicking::try

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:447:19

  24: std::panic::catch_unwind

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panic.rs:137:14

  25: winit::platform_impl::platform::event_loop::stop_app_on_panic

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/event_loop.rs:265:11

  26: winit::platform_impl::platform::observer::control_flow_handler

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/observer.rs:143:5

  27: winit::platform_impl::platform::observer::control_flow_end_handler

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/observer.rs:179:9

  28: 

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  37: 

  38: <() as objc::message::MessageArguments>::invoke

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/objc-0.2.7/src/message/mod.rs:128:17

  39: objc::message::platform::send_unverified

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/objc-0.2.7/src/message/apple/mod.rs:27:9

  40: objc::message::send_message

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/objc-0.2.7/src/message/mod.rs:178:5

  41: winit::platform_impl::platform::event_loop::EventLoop::run_return::{{closure}}

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/objc-0.2.7/src/macros.rs:133:15

  42: objc::rc::autorelease::autoreleasepool

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/objc-0.2.7/src/rc/autorelease.rs:29:5

  43: winit::platform_impl::platform::event_loop::EventLoop::run_return

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/event_loop.rs:212:25

  44: winit::platform_impl::platform::event_loop::EventLoop::run

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/platform_impl/macos/event_loop.rs:191:25

  45: winit::event_loop::EventLoop::run

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.27.5/src/event_loop.rs:278:9

  46: bevy_winit::run

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_winit-0.9.1/src/lib.rs:263:5

  47: bevy_winit::winit_runner_with

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_winit-0.9.1/src/lib.rs:645:9

  48: bevy_winit::winit_runner

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_winit-0.9.1/src/lib.rs:303:5

  49: core::ops::function::Fn::call

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/ops/function.rs:78:5

  50:  as core::ops::function::Fn>::call

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/alloc/src/boxed.rs:2001:9

  51: bevy_app::app::App::run

             at /Users/matrixdev/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_app-0.9.1/src/app.rs:168:9

  52: fleet_deck::main

             at ./src/main.rs:63:5

  53: core::ops::function::FnOnce::call_once

             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/ops/function.rs:251:5

## Suggestion

It probably happens when one system despawns a parent while other adds a child. In such cases I think child should be despawned automatically if parent was not found.

Also maybe it is better to use `get_entity_mut` with `if let` instead of `entity_mut` to avoid such panics?