The important part here is: Caused by: java.lang.NumberFormatException: null
and below: at java.lang.Integer.parseInt(Integer.java:454)
and, somehow, below: at button2.serialEvent(button2.java:143)

What does it mean?
Going, up, we see that in your sketch named button2, you have a function named serialEvent, which is used to handle the event coming from the serial interface. The remainder of the stack trace also shows it is called by reflection, by processing.serial.Serial.serialEvent but that's less important.
Then, the Integer.parseInt function is called, and throwing a NumberFormatException. Because it is called with null.

In clear, you are probably doing something like Integer.parseInt(someVar) and this someVar is null. (Or using unbinary(someVar), that's the only call of this function in Processing that isn't protected against this error.)
Perhaps trying to convert some number transmitted via the serial interface, but it is null, for some reason.

Either you ensure no null is transmitted, or you check if the value is not null before calling the function.

PS.: Take a look at the forum descriptions, I moved the topic. See also the guide.