This bellow line is throwing an NPE, producing an IllegalStateException in the exception chain:
int stopPortRange = Integer.parseInt(stopPort.getText().toString());
as you don't initialize stopPort (or startPort) in your onCreate method. You should have something like:
stopPort = (EditText) findViewById(R.id.stop_port_field);
simply declares the variable which is null by default. Only instances that have been instantiated can have their methods invoked.