Don't panic when getting refresh rate failed (#2533)

This fixes a crash on macOS when trying to get the monitor
refresh rate from the disabled monitor.

Co-authored-by: Jet Spark <lixiaopeng.jetspark@bytedance.com>
Co-authored-by: Mads Marquart <mads@marquart.dk>

src/monitor.rs

@@ -143,6 +143,9 @@ impl MonitorHandle {
 
     /// The monitor refresh rate used by the system.
     ///
+    /// Return `Some` if succeed, or `None` if failed, which usually happens when the monitor
+    /// the window is on is removed.
+    ///
     /// When using exclusive fullscreen, the refresh rate of the [`VideoMode`] that was used to
     /// enter fullscreen should be used instead.
     #[inline]