If you check out that stack trace, you see that the top level exception is a JSONException (line 2). That's caused by an InvocationTargetException (line 70). That in turn is caused by a NullPointerException (line 77). That's what you should focus on.
This tells you that the NullPointerException is thrown by method execute of class com.cashmate.struts.Release_EditVouchers, on line 48 of file Release_EditVouchers.java. That's now the number one piece of code to focus on. Why don't you post that line here; if it's not too long the entire execute method.