Migrated package to null safety

lib/app_bar.dart

@@ -55,7 +55,7 @@ class BackdropAppBar extends StatelessWidget implements PreferredSizeWidget {
   /// If this is `null` and if [BackdropAppBar.automaticallyImplyLeading] is
   /// set to `true`, [BackdropAppBar] sets the underlying [AppBar.leading] to
   /// [BackdropToggleButton].
-  final Widget leading;
+  final Widget? leading;
 
   /// See [AppBar.automaticallyImplyLeading].
   ///
@@ -67,45 +67,45 @@ class BackdropAppBar extends StatelessWidget implements PreferredSizeWidget {
   final bool automaticallyImplyLeading;
 
   /// The widget that should be displayed as the [AppBar] title.
-  final Widget title;
+  final Widget? title;
 
   /// See [AppBar.actions].
-  final List<Widget> actions;
+  final List<Widget>? actions;
 
   /// See [AppBar.flexibleSpace].
-  final Widget flexibleSpace;
+  final Widget? flexibleSpace;
 
   /// See [AppBar.bottom].
-  final PreferredSizeWidget bottom;
+  final PreferredSizeWidget? bottom;
 
   /// See [AppBar.elevation].
   ///
   /// Defaults to 0.0. This differs from [AppBar.elevation].
   final double elevation;
 
   /// See [AppBar.shape]
-  final ShapeBorder shape;
+  final ShapeBorder? shape;
 
   /// See [AppBar.backgroundColor].
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// See [AppBar.brightness].
-  final Brightness brightness;
+  final Brightness? brightness;
 
   /// See [AppBar.iconTheme].
-  final IconThemeData iconTheme;
+  final IconThemeData? iconTheme;
 
   /// See [AppBar.actionsIconTheme].
-  final IconThemeData actionsIconTheme;
+  final IconThemeData? actionsIconTheme;
 
   /// See [AppBar.textTheme].
-  final TextTheme textTheme;
+  final TextTheme? textTheme;
 
   /// See [AppBar.primary].
   final bool primary;
 
   /// See [AppBar.centerTitle].
-  final bool centerTitle;
+  final bool? centerTitle;
 
   /// See [AppBar.excludeHeaderSemantics].
   final bool excludeHeaderSemantics;
@@ -127,7 +127,7 @@ class BackdropAppBar extends StatelessWidget implements PreferredSizeWidget {
   ///
   /// For more information see [AppBar].
   BackdropAppBar({
-    Key key,
+    Key? key,
     this.leading,
     this.automaticallyImplyLeading = true,
     this.title,

lib/button.dart

@@ -49,7 +49,7 @@ class BackdropToggleButton extends StatelessWidget {
       icon: AnimatedIcon(
         icon: icon,
         color: color,
-        progress: Backdrop.of(context).animationController.view,
+        progress: Backdrop.of(context).animationController!.view,
       ),
       onPressed: () => Backdrop.of(context).fling(),
     );

lib/navigation.dart

@@ -43,33 +43,33 @@ class BackdropNavigationBackLayer extends StatelessWidget {
   final List<Widget> items;
 
   /// Callback that is called whenever a list item is tapped by the user.
-  final ValueChanged<int> onTap;
+  final ValueChanged<int>? onTap;
 
   /// Customizable separator used with [ListView.separated].
   @Deprecated("Replace by use of `separatorBuilder`."
       "This feature was deprecated after v0.4.2.")
-  final Widget separator;
+  final Widget? separator;
 
   /// Customizable separatorBuilder used with [ListView.separated].
-  final IndexedWidgetBuilder separatorBuilder;
+  final IndexedWidgetBuilder? separatorBuilder;
 
   /// Allows to set a [Padding] for each item of the list.
-  final EdgeInsetsGeometry itemPadding;
+  final EdgeInsetsGeometry? itemPadding;
 
   /// Sets a custom border on the list items' [InkWell].
   /// See [InkResponse.customBorder].
-  final ShapeBorder itemSplashBorder;
+  final ShapeBorder? itemSplashBorder;
 
   /// Sets a custom splash color on the list items. See [InkResponse.splashColor].
-  final Color itemSplashColor;
+  final Color? itemSplashColor;
 
   /// Creates an instance of [BackdropNavigationBackLayer] to be used with
   /// [BackdropScaffold].
   ///
   /// The argument [items] is required and must not be `null` and not empty.
   BackdropNavigationBackLayer({
-    Key key,
-    @required this.items,
+    Key? key,
+    required this.items,
     this.onTap,
     this.separator,
     this.separatorBuilder,