Unable to 'forward ' the flutter animation

I have this code which uses an AnimationController and Tween to rotate a container along Z-axis when Start/Stop Button is pressed. The problem is it forwards the animation only first time after that no matter how many times I press the button it does nothing and the container remains stationary .

curvesanimation.dart which has all the animation logic

import 'package:flutter/material.dart';
import 'dart:math' as math;


class CurvedAnimationExample extends StatefulWidget {
  CurvedAnimationExample({Key key}) : super(key: key);
  @override
  CurvedAnimationExampleState createState() => CurvedAnimationExampleState();
}

class CurvedAnimationExampleState extends State<CurvedAnimationExample>
    with SingleTickerProviderStateMixin {
  Animation<double> _animation;
  AnimationController _animationController;

  @override
  void initState() {
    super.initState();

    _animationController =
        AnimationController(vsync: this, duration: Duration(seconds: 5))
          ..addListener(() {
            setState(() {});
          });

    final Animation curve =
        CurvedAnimation(parent: _animationController, curve: Curves.easeIn);

    _animation = Tween<double>(begin: 0, end: math.pi * 2).animate(curve);
  }

  void startStopAnimation() {                      
    print(_animationController.isAnimating.toString());
    if (_animationController.isAnimating)
      _animationController.stop();
    else {
      _animationController.forward();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Center(
        child: Column(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        Transform(
          alignment: Alignment.center,
          transform: Matrix4.identity()..rotateZ(_animation.value),
          child: Container(
            color: Colors.pinkAccent,
            width: 200,
            height: 200,
          ),
        ),
        RaisedButton(
          child: const Text("Start/Stop Animation"),
          elevation: 15,
          color: Colors.blueGrey,
          onPressed: () => startStopAnimation(),
        )
      ],
    ));
  }

  @override
  void dispose() {
    _animationController.dispose();
    print("Curved Animation Example : dispose is called");
    super.dispose();
  }
}

main.dart file

import 'package:flutter/material.dart';
import 'curvesanimations.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomeScaffold(
        title: "Curved Animation Example",
      ),
    );
  }
}

class MyHomeScaffold extends StatelessWidget {
  final String title;
  MyHomeScaffold({this.title});
  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: CurvedAnimationExample(),
      ),
    );
  }
}