Fixed extremely rare race condition in DiscoveryManager

Author: cmaglie

Diff for: internal/arduino/discovery/discoverymanager/discoverymanager.go

@@ -171,11 +171,16 @@ func (dm *DiscoveryManager) Watch() (*PortWatcher, error) {
 		defer dm.watchersMutex.Unlock()
 		delete(dm.watchers, watcher)
 		close(watcher.feed)
+		watcher.feed = nil
 	}
 	go func() {
 		dm.watchersMutex.Lock()
 		defer dm.watchersMutex.Unlock()
 
+		if watcher.feed == nil {
+			return
+		}
+
 		// if watcher is still online...
 		if _, ok := dm.watchers[watcher]; ok {
 			return