diff --git a/.github/workflows/flutter_analyze.yml b/.github/workflows/flutter_analyze.yml
index efafd499..4a0d8c5e 100644
--- a/.github/workflows/flutter_analyze.yml
+++ b/.github/workflows/flutter_analyze.yml
@@ -23,5 +23,8 @@ jobs:
       - name: Install dependencies
         run: flutter pub get
 
+      - name: Formatting check
+        run: flutter format . --dry-run --set-exit-if-changed
+
       - name: Analyse
         run: flutter analyze
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 00000000..c4f39f56
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,9 @@
+# Way to contribute
+
+1. Fork the repo and create your branch from `master` or `devlop`.
+2. Clone the project to your own machine.
+3. Commit changes to your own branch
+4. Make sure your code lints.
+5. Push your work back up to your fork.
+6. Create a pull request on `devlop` branch.
+7. And Done!
\ No newline at end of file
diff --git a/flutter_calendar_page.iml b/flutter_calendar_page.iml
deleted file mode 100644
index a461e815..00000000
--- a/flutter_calendar_page.iml
+++ /dev/null
@@ -1,14 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<module type="JAVA_MODULE" version="4">
-  <component name="NewModuleRootManager" inherit-compiler-output="true">
-    <exclude-output />
-    <content url="file://$MODULE_DIR$">
-      <excludeFolder url="file://$MODULE_DIR$/.dart_tool" />
-      <excludeFolder url="file://$MODULE_DIR$/.pub" />
-      <excludeFolder url="file://$MODULE_DIR$/build" />
-    </content>
-    <orderEntry type="sourceFolder" forTests="false" />
-    <orderEntry type="library" name="Dart SDK" level="project" />
-    <orderEntry type="library" name="Dart Packages" level="project" />
-  </component>
-</module>
\ No newline at end of file
diff --git a/lib/src/day_view/day_view.dart b/lib/src/day_view/day_view.dart
index 12ca63e6..da7a1669 100644
--- a/lib/src/day_view/day_view.dart
+++ b/lib/src/day_view/day_view.dart
@@ -213,12 +213,10 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
 
   late DateWidgetBuilder _dayTitleBuilder;
 
-  late EventController<T> _controller;
+  EventController<T>? _controller;
 
   late ScrollController _scrollController;
 
-  bool _controllerAdded = false;
-
   late VoidCallback _reloadCallback;
 
   final _scrollConfiguration = EventScrollConfiguration<T>();
@@ -249,10 +247,10 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
     final newController = widget.controller ??
         CalendarControllerProvider.of<T>(context).controller;
 
-    if (_controller != newController) {
+    if (newController != _controller) {
       _controller = newController;
 
-      _controller
+      _controller!
         // Removes existing callback.
         ..removeListener(_reloadCallback)
 
@@ -261,8 +259,6 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
         ..addListener(_reloadCallback);
     }
 
-    _controllerAdded = true;
-
     _updateViewDimensions();
   }
 
@@ -274,9 +270,9 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
         CalendarControllerProvider.of<T>(context).controller;
 
     if (newController != _controller) {
-      _controller.removeListener(_reloadCallback);
+      _controller?.removeListener(_reloadCallback);
       _controller = newController;
-      _controller.addListener(_reloadCallback);
+      _controller?.addListener(_reloadCallback);
     }
 
     // Update date range.
@@ -301,7 +297,7 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
 
   @override
   void dispose() {
-    _controller.removeListener(_reloadCallback);
+    _controller?.removeListener(_reloadCallback);
     _pageController.dispose();
     super.dispose();
   }
@@ -356,7 +352,7 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
                                   verticalLineOffset: widget.verticalLineOffset,
                                   showVerticalLine: widget.showVerticalLine,
                                   height: _height,
-                                  controller: _controller,
+                                  controller: controller,
                                   hourHeight: _hourHeight,
                                   eventArranger: _eventArranger,
                                   minuteSlotSize: widget.minuteSlotSize,
@@ -379,9 +375,11 @@ class DayViewState<T extends Object?> extends State<DayView<T>> {
   /// This will throw [AssertionError] if controller is called before its
   /// initialization is complete.
   EventController<T> get controller {
-    assert(_controllerAdded, "EventController is not initialized yet.");
+    if (_controller == null) {
+      throw "EventController is not initialized yet.";
+    }
 
-    return _controller;
+    return _controller!;
   }
 
   /// Reloads page.
diff --git a/lib/src/month_view/month_view.dart b/lib/src/month_view/month_view.dart
index 3c3e3bb2..056aa42f 100644
--- a/lib/src/month_view/month_view.dart
+++ b/lib/src/month_view/month_view.dart
@@ -167,9 +167,7 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
 
   late DateWidgetBuilder _headerBuilder;
 
-  late EventController<T> _controller;
-
-  bool _controllerAdded = false;
+  EventController<T>? _controller;
 
   late VoidCallback _reloadCallback;
 
@@ -199,10 +197,10 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
     final newController = widget.controller ??
         CalendarControllerProvider.of<T>(context).controller;
 
-    if (_controller != newController) {
+    if (newController != _controller) {
       _controller = newController;
 
-      _controller
+      _controller!
         // Removes existing callback.
         ..removeListener(_reloadCallback)
 
@@ -211,8 +209,6 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
         ..addListener(_reloadCallback);
     }
 
-    _controllerAdded = true;
-
     updateViewDimensions();
   }
 
@@ -224,9 +220,9 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
         CalendarControllerProvider.of<T>(context).controller;
 
     if (newController != _controller) {
-      _controller.removeListener(_reloadCallback);
+      _controller?.removeListener(_reloadCallback);
       _controller = newController;
-      _controller.addListener(_reloadCallback);
+      _controller?.addListener(_reloadCallback);
     }
 
     // Update date range.
@@ -246,7 +242,7 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
 
   @override
   void dispose() {
-    _controller.removeListener(_reloadCallback);
+    _controller?.removeListener(_reloadCallback);
     _pageController.dispose();
     super.dispose();
   }
@@ -301,7 +297,7 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
                               onDateLongPress: widget.onDateLongPress,
                               width: _width,
                               height: _height,
-                              controller: _controller,
+                              controller: controller,
                               borderColor: widget.borderColor,
                               borderSize: widget.borderSize,
                               cellBuilder: _cellBuilder,
@@ -330,9 +326,11 @@ class MonthViewState<T extends Object?> extends State<MonthView<T>> {
   /// This will throw [AssertionError] if controller is called before its
   /// initialization is complete.
   EventController<T> get controller {
-    assert(_controllerAdded, "EventController is not initialized yet.");
+    if (_controller == null) {
+      throw "EventController is not initialized yet.";
+    }
 
-    return _controller;
+    return _controller!;
   }
 
   void _reload() {
diff --git a/lib/src/week_view/week_view.dart b/lib/src/week_view/week_view.dart
index cc25f310..2f20a13c 100644
--- a/lib/src/week_view/week_view.dart
+++ b/lib/src/week_view/week_view.dart
@@ -211,11 +211,9 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
   late double _weekTitleWidth;
   late int _totalDaysInWeek;
 
-  bool _controllerAdded = false;
-
   late VoidCallback _reloadCallback;
 
-  late EventController<T> _controller;
+  EventController<T>? _controller;
 
   late ScrollController _scrollController;
   late List<WeekDays> _weekDays;
@@ -254,7 +252,7 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
     if (_controller != newController) {
       _controller = newController;
 
-      _controller
+      _controller!
         // Removes existing callback.
         ..removeListener(_reloadCallback)
 
@@ -263,8 +261,6 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
         ..addListener(_reloadCallback);
     }
 
-    _controllerAdded = true;
-
     _updateViewDimensions();
   }
 
@@ -276,9 +272,9 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
         CalendarControllerProvider.of<T>(context).controller;
 
     if (newController != _controller) {
-      _controller.removeListener(_reloadCallback);
+      _controller?.removeListener(_reloadCallback);
       _controller = newController;
-      _controller.addListener(_reloadCallback);
+      _controller?.addListener(_reloadCallback);
     }
 
     _setWeekDays();
@@ -305,7 +301,7 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
 
   @override
   void dispose() {
-    _controller.removeListener(_reloadCallback);
+    _controller?.removeListener(_reloadCallback);
     _pageController.dispose();
     super.dispose();
   }
@@ -363,12 +359,12 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
                                 timeLineWidth: _timeLineWidth,
                                 verticalLineOffset: 0,
                                 showVerticalLine: true,
-                                controller: _controller,
+                                controller: controller,
                                 hourHeight: _hourHeight,
                                 scrollController: _scrollController,
                                 eventArranger: _eventArranger,
                                 weekDays: _weekDays,
-                            minuteSlotSize: widget.minuteSlotSize,
+                                minuteSlotSize: widget.minuteSlotSize,
                                 scrollConfiguration: _scrollConfiguration,
                               ));
                     },
@@ -387,9 +383,11 @@ class WeekViewState<T extends Object?> extends State<WeekView<T>> {
   /// This will throw [AssertionError] if controller is called before its
   /// initialization is complete.
   EventController<T> get controller {
-    assert(_controllerAdded, "EventController is not initialized yet.");
+    if (_controller == null) {
+      throw "EventController is not initialized yet.";
+    }
 
-    return _controller;
+    return _controller!;
   }
 
   /// Reloads page.