title: Failed to handle method call on local notification
body: <p>I'm testing out this <a href="https://pub.dev/packages/flutter_local_notifications#-readme-tab-" rel="noreferrer">this</a> plugin for local notifications. I want to show a notification on a button press, this is my code:</p>
  
  <pre><code>FlatButton(
              onPressed: () {
                FlutterLocalNotificationsPlugin localNotifPlugin =
                    new FlutterLocalNotificationsPlugin();
                var androidChannelSpecifics = AndroidNotificationDetails(
                    'default',
                    'Test App Notifications',
                    'Channel for default notifications from test app');
                var iOSChannelSpecifics = IOSNotificationDetails();
                var platformChannelSpecifics = NotificationDetails(
                    androidChannelSpecifics, iOSChannelSpecifics);
                localNotifPlugin.show(
                    0,
                    "Test Notificaiton",
                    "This is your test notification! :)",
                    platformChannelSpecifics);
              },
              child: Text("Show Notification"),
            )
  </code></pre>
  
  <p>This is very similar to the documentation for the plugin but its not working and I'm getting this error when I press the button: </p>
  
  <pre><code>    D/EGL_emulation(24401): eglMakeCurrent: 0xa80053c0: ver 2 0 (tinfo 0xa8003360)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401): Failed to handle method call
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401): java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin.setSmallIcon(FlutterLocalNotificationsPlugin.java:156)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin.createNotification(FlutterLocalNotificationsPlugin.java:131)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin.showNotification(FlutterLocalNotificationsPlugin.java:598)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin.show(FlutterLocalNotificationsPlugin.java:690)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin.onMethodCall(FlutterLocalNotificationsPlugin.java:619)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:222)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:96)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:656)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at android.os.MessageQueue.nativePollOnce(Native Method)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at android.os.MessageQueue.next(MessageQueue.java:323)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at android.os.Looper.loop(Looper.java:136)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at android.app.ActivityThread.main(ActivityThread.java:6077)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at java.lang.reflect.Method.invoke(Native Method)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
  E/MethodChannel#dexterous.com/flutter/local_notifications(24401):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
  E/flutter (24401): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: PlatformException(error, Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference, null)
  E/flutter (24401): #0      StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:569:7)
  E/flutter (24401): #1      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:316:33)
  E/flutter (24401): &lt;asynchronous suspension&gt;
  E/flutter (24401): #2      FlutterLocalNotificationsPlugin.show (package:flutter_local_notifications/src/flutter_local_notifications.dart:120:20)
  E/flutter (24401): &lt;asynchronous suspension&gt;
  E/flutter (24401): #3      HomePage.build.&lt;anonymous closure&gt; (package:provider_test/main.dart:53:36)
  E/flutter (24401): #4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:654:14)
  E/flutter (24401): #5      _InkResponseState.build.&lt;anonymous closure&gt; (package:flutter/src/material/ink_well.dart:729:32)
  E/flutter (24401): #6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
  E/flutter (24401): #7      TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:365:11)
  E/flutter (24401): #8      TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:275:7)
  E/flutter (24401): #9      PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:455:9)
  E/flutter (24401): #10     PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:75:13)
  E/flutter (24401): #11     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:102:11)
  E/flutter (24401): #12     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:218:19)
  E/flutter (24401): #13     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
  E/flutter (24401): #14     GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
  E/flutter (24401): #15     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
  E/flutter (24401): #16     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
  E/flutter (24401): #17     _rootRunUnary (dart:async/zone.dart:1136:13)
  E/flutter (24401): #18     _CustomZone.runUnary (dart:async/zone.dart:1029:19)
  E/flutter (24401): #19     _CustomZone.runUnaryGuarded (dart:async/zone.dart:931:7)
  E/flutter (24401): #20     _invoke1 (dart:ui/hooks.dart:263:10)
  E/flutter (24401): #21     _dispatchPointerDataPacket (dart:ui/hooks.dart:172:5)
  </code></pre>
  
  <p>What did I do wrong?</p>