src/main/java/org/jivesoftware/smack/util/collections/AbstractReferenceMap.java
public V remove(Object key) {
        if (key == null) {
            return null;
        }
        purgeBeforeWrite();
        return super.remove(key);
    }
public V remove(Object key) {
        if (key == null) {
            return null;
        }
        purgeBeforeWrite();
        return super.remove(key);
    }
Removes the specified mapping from this map.

@param key
the mapping to remove
@return the value mapped to the removed key, null if key not in map