Fix bugs / errors in Android SIP application

The task is to fix the following bugs / errors in an Android SIP application: 1. All occurrences of 'Fatal signal 11' error causing crash on incoming / outgoing SIP calls on Android 4.x and 5.x: A/libc(1496): Fatal signal 11 (SIGSEGV) at 0xdeadd00d (code=1) A/libc(27466): Fatal signal 11 (SIGSEGV), code 1, fault addr 0x39 in tid 27863 (AsyncTask #3) 2. All occurrences of 'Fatal signal 6' error causing crash on incoming / outgoing SIP calls on Android 4.x and 5.x: A/libc(20362): Fatal signal 6 (SIGABRT) at 0x00004f8a (code=-6), thread 20496 (Thread-11014) A/libc(26922): Fatal signal 6 (SIGABRT) at 0x0000692a (code=-6), thread 27297 (AsyncTask #3) 3. All occurrences of 'NullPointerException' error in Android 4.x and 5.x 4. Fix a bug in Android 5.x which causes the 'Call Ended' screen to be displayed before a popup on Samsung Galaxy S5 phone.