How to handle null point exception while using switch strings

Was able to launch and close browser 1 only.
            
Error:


java.lang.NullPointerException
      at java.util.Hashtable.put(Unknown Source)
      at java.util.Properties.setProperty(Unknown Source)
      at java.lang.System.setProperty(Unknown Source)
      at newpackage.Launchbrowsers.Browser(Launchbrowsers.java:32)
      at newpackage.Launchbrowsers.main(Launchbrowsers.java:13)

Thanks in advance for any help.