I see this in two places in Submit:

String line = in.readLine().trim();
while (line != null & !line.equals("OK")) {

Note that because of the use of trim() in the declaration, line will never be null. in.readLine() may return null, but the trim() call will force a NullPointerException when that happens, and the try-catch doesn't handle that.

Solution - test the return value of in.readLine() before doing anything, such as trim().