With using alloc this throws an error:
```rust
static POLL_FN: Mutex<RefCell<Option<Box<dyn FnMut() + 'static>>>> = Mutex::new(RefCell::new(None));
```
```
`(dyn FnMut() + 'static)` cannot be sent between threads safely
the trait `Send` is not implemented for `(dyn FnMut() + 'static)`, which is required by `critical_section::Mutex<RefCell<Option<Box<(dyn FnMut() + 'static)>>>>: Sync`
```