You are getting NullPointerException as the "output" is null when the while loop ends. You can collect the output in some buffer and then use it, something like this-
StringBuilder buffer = new StringBuilder();
String output;
System.out.println("Output from Server .... \n");
while ((output = br.readLine()) != null) {
System.out.println(output);
buffer.append(output);
}
output = buffer.toString(); // now you have the output
conn.disconnect();