Fixed crash when unregistering EventBus subsribers

closes #291

android/app/src/main/java/com/reactnativenavigation/events/EventBus.java

 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.ListIterator;
 
 public enum EventBus {
     instance;
     }
 
     public void unregister(Subscriber subscriber) {
-        for (WeakReference<Subscriber> ref : subscribers) {
+        ListIterator<WeakReference<Subscriber>> iterator = subscribers.listIterator();
+        while (iterator.hasNext()) {
+            WeakReference<Subscriber> ref = iterator.next();
             Subscriber registered = ref.get();
             if (registered != null && registered == subscriber) {
                 subscribers.remove(ref);