The myNex.NextionListen(); has a delay in it. So your sketch can not respond quickly.
Other functions for the Nextion wait for a reply.
I'm not sure what the myNex.readNumber() is doing, it is probably up to no good as well.
Do you use the normal Serial port for the Nextion ? Please use the normal Serial port for debug messages to the Serial Monitor and use one of the others for the Nextion display.