Um...the message is pretty explicit:
JOptionPane: type must be one of JOptionPane.ERROR_MESSAGE, JOptionPane.INFORMATION_MESSAGE, JOptionPane.WARNING_MESSAGE, JOptionPane.QUESTION_MESSAGE or JOptionPane.PLAIN_MESSAGE
So look at your code (btnmathActionPerformed would be a good place to start) and see exactly what you are doing with the JOptionPane.
We can't - we have no access to your screen, HDD, or data, and we can't read your mind!