Exception about removeEventListener

[HughWick]

I tried to remove the EventListener after getting the serialPort object in another class and found an exception, the final reason is that I did not add the EventListener beforehand, I tried to use the getEventListener (there is no such method)
or
the getEventListenerAdded flag

//but the variable is private
 private boolean eventListenerAdded = false;

to get if there is currently an EventListener event.

Is there something I'm doing wrong?

Can you add a flag function to get whether there is currently an EventListener in the next version