title: Android Alarm Manager callback doesn't work
body: <p>Im trying to use plugin <a href="https://pub.dartlang.org/packages/android_alarm_manager" rel="nofollow noreferrer">Android Alarm Manager</a> for timer and code execution in background, but cant really get it working right. If i set something like "print("v")" as a callback - everything works fine, but when Im trying to do something extra, it just doesn't work.</p>
  
  <pre class="lang-dart prettyprint-override"><code>import 'package:flutter/material.dart';
  import 'dart:async';
  import 'package:android_alarm_manager/android_alarm_manager.dart';
  import 'dart:isolate';
  
  class MyApp extends StatefulWidget {
    _MyAppState createState() =&gt; _MyAppState();
  }
  
  
  
  startTimer(sendport) async {
    await AndroidAlarmManager.oneShot(
        Duration(seconds: 60), 0, timerCallback(sendport),
        wakeup: true, exact: true);
  }
  
  timerCallback(sendport) {
    sendport.send("DONE");
  }
  
  class _MyAppState extends State&lt;MyApp&gt; {
    ReceivePort receivePort = ReceivePort();
    SendPort sendport;
  
    @override
    void initState() {
      super.initState();
      AndroidAlarmManager.initialize();
      receivePort.listen((v) {
        print(v);
      });
    }
  
    @override
    Widget build(BuildContext context) {
      RaisedButton(
        onPressed: startTimer(sendport),
        child: Text("Start"),
      );
    }
  }
  </code></pre>
  
  <p>I expect that code to send message after 1 minute, instead im getting message right after execution and get Error
  "/flutter (11424): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: 'dart:ui/plugins.dart': Failed assertion: line 62: '': 'callback' must not be null."</p>