fix null safety

android/app/build.gradle

@@ -24,12 +24,16 @@ if (flutterVersionName == null) {
 apply plugin: 'com.android.application'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
+def keystoreProperties = new Properties()
+def keystorePropertiesFile = rootProject.file('key.properties')
+if (keystorePropertiesFile.exists()) {
+    keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
+}
+
 android {
     compileSdkVersion 30
 
     defaultConfig {
-        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
-        applicationId "com.mokkon.fcs.fcs"
         minSdkVersion 21
         targetSdkVersion 30
         versionCode flutterVersionCode.toInteger()