fix: Added null-check to unregisterListener

Author: julian-siebert

plugin/src/main/java/studio/o7/octopus/plugin/OctopusImpl.java

@@ -117,8 +117,11 @@ public void unregisterListener(@NonNull Listener listener) {
     @Override
     public void unregisterListener(@NonNull UUID listenerUniqueId) {
         var pair = this.listeners.get(listenerUniqueId);
-        pair.right().onCompleted();
+        if (pair == null) return;
         this.listeners.remove(listenerUniqueId);
+        var right = pair.right();
+        if (right == null) return;
+        right.onCompleted();
     }
 
     @Override