diff --git a/android/app/build.gradle b/android/app/build.gradle
index acfa202..010a5e0 100644
--- a/android/app/build.gradle
+++ b/android/app/build.gradle
@@ -72,5 +72,5 @@ dependencies {
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'androidx.test:runner:1.1.0-alpha4'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0-alpha4'
-    compile 'com.android.support:multidex:1.0.3'
+    implementation 'com.android.support:multidex:1.0.3'
 }
diff --git a/android/build.gradle b/android/build.gradle
index 600217c..01316da 100644
--- a/android/build.gradle
+++ b/android/build.gradle
@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:4.2.0'
+        classpath 'com.android.tools.build:gradle:7.3.1'
     }
 }
 
diff --git a/lib/screens/home.dart b/lib/screens/home.dart
index 5abf709..d75b43a 100644
--- a/lib/screens/home.dart
+++ b/lib/screens/home.dart
@@ -122,7 +122,7 @@ class HomeState extends State<Home> {
     return null;
   }
 
-  void _handleSubmitted(BuildContext context, Subject subject, String title,
+  void _handleSubmitted(BuildContext context, Subject? subject, String title,
       String description) async {
     updateModel(
         context,
diff --git a/lib/screens/subject_details.dart b/lib/screens/subject_details.dart
index ddc9df1..4c58023 100644
--- a/lib/screens/subject_details.dart
+++ b/lib/screens/subject_details.dart
@@ -158,10 +158,10 @@ class SubjectDetailsState extends State<SubjectDetails> {
     return null;
   }
 
-  void _handleEdit(BuildContext context, Subject subject, String title,
+  void _handleEdit(BuildContext context, Subject? subject, String title,
       String description) async {
     updateModel(context, (model) {
-      Subject s = model.subjects[model.subjects.indexOf(subject)];
+      Subject s = model.subjects[model.subjects.indexOf(subject!)];
       if (title.isNotEmpty) {
         s.title = title;
       }
@@ -180,11 +180,11 @@ class SubjectDetailsState extends State<SubjectDetails> {
     return null;
   }
 
-  void _handleSubmitted(BuildContext context, Subject subject, String title,
+  void _handleSubmitted(BuildContext context, Subject? subject, String title,
       String description) async {
     updateModel(
         context,
-        (model) => model.addTopic(subject,
+        (model) => model.addTopic(subject!,
             new Topic(title: title, description: description, indexes: [])));
     createDirs(title);
     Navigator.pop(context);
diff --git a/lib/screens/topic_details.dart b/lib/screens/topic_details.dart
index f76faaa..24b1fb9 100644
--- a/lib/screens/topic_details.dart
+++ b/lib/screens/topic_details.dart
@@ -230,10 +230,10 @@ class TopicDetailsState extends State<TopicDetails> {
     return null;
   }
 
-  void _handleEdit(BuildContext context, Subject subject, String title,
+  void _handleEdit(BuildContext context, Subject? subject, String title,
       String description) async {
     updateModel(context, (model) {
-      Subject s = model.subjects[model.subjects.indexOf(subject)];
+      Subject s = model.subjects[model.subjects.indexOf(subject!)];
       Topic t = s.topics[s.topics.indexOf(widget.topic)];
       if (title.isNotEmpty) {
         String oldTitle = t.title;
diff --git a/lib/widgets/add_topic_form.dart b/lib/widgets/add_topic_form.dart
index 51af380..4feef99 100644
--- a/lib/widgets/add_topic_form.dart
+++ b/lib/widgets/add_topic_form.dart
@@ -5,7 +5,7 @@ import 'package:study_snap/models/subject.dart';
 typedef String? ValidateCallback(
     BuildContext context, Subject? subject, String? value);
 typedef void SubmitCallback(
-    BuildContext context, Subject subject, String title, String description);
+    BuildContext context, Subject? subject, String title, String description);
 
 class AddTopicForm extends StatefulWidget {
   final Subject? subject;
@@ -80,7 +80,7 @@ class AddTopicFormState extends State<AddTopicForm> {
                 if (form != null && form.validate()) {
                   form.save();
                   widget.handleSubmitted(
-                      context, widget.subject!, title, description);
+                      context, widget.subject, title, description);
                 }
               },
             ),
diff --git a/pubspec.yaml b/pubspec.yaml
index 07d3e7a..89ac089 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -11,7 +11,7 @@ description: Flutter app for managing study materials in form of photos.
 # In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
 # Read more about iOS versioning at
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
-version: 1.1.1+5
+version: 1.1.1+6
 
 environment:
   sdk: ">=2.12.0 <3.0.0"