A continuación pego el LogCat.
01-19 14:01:03.584 1897-1897/jsonconnect.josue.es.jsonconnect I/System.out﹕ waiting for debugger to settle...
01-19 14:01:03.796 1897-1897/jsonconnect.josue.es.jsonconnect I/System.out﹕ waiting for debugger to settle...
01-19 14:01:04.015 1897-1897/jsonconnect.josue.es.jsonconnect I/System.out﹕ waiting for debugger to settle...
01-19 14:01:04.239 1897-1897/jsonconnect.josue.es.jsonconnect I/System.out﹕ waiting for debugger to settle...
01-19 14:01:04.455 1897-1897/jsonconnect.josue.es.jsonconnect I/System.out﹕ debugger has settled (1354)
01-19 14:01:04.593 1897-1897/jsonconnect.josue.es.jsonconnect E/mensaje log﹕ Error en 1º try android.os.NetworkOnMainThreadException
01-19 14:01:04.593 1897-1897/jsonconnect.josue.es.jsonconnect D/AndroidRuntime﹕ Shutting down VM
01-19 14:01:04.624 1897-1897/jsonconnect.josue.es.jsonconnect E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: jsonconnect.josue.es.jsonconnect, PID: 1897
java.lang.RuntimeException: Unable to start activity ComponentInfo{jsonconnect.josue.es.jsonconnect/jsonconnect.josue.es.jsonconnect.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
at org.json.JSONTokener.nextValue(JSONTokener.java:94)
at org.json.JSONObject.<init>(JSONObject.java:156)
at org.json.JSONObject.<init>(JSONObject.java:173)
at jsonconnect.josue.es.jsonconnect.MainActivity.onCreate(MainActivity.java:79)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-19 14:06:04.994 1897-1897/jsonconnect.josue.es.jsonconnect I/Process﹕ Sending signal. PID: 1897 SIG: 9