I didn't change anything, I followed the instruction and ran the code, below is the log.
โ Built build\app\outputs\flutter-apk\app-debug.apk.
E/AndroidRuntime(10780): FATAL EXCEPTION: main
E/AndroidRuntime(10780): Process: com.mindle.inventory.merchant, PID: 10780
E/AndroidRuntime(10780): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.mindle.inventory.merchant/com.mindle.inventory.merchant.MainActivity}: java.lang.ClassNotFoundException: Didn't find class "com.mindle.inventory.merchant.MainActivity" on path: DexPathList[[zip file "/data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/base.apk"],nativeLibraryDirectories=[/data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/lib/x86, /data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/base.apk!/lib/x86, /system/lib, /system/product/lib]]
E/AndroidRuntime(10780): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
E/AndroidRuntime(10780): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
E/AndroidRuntime(10780): at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
E/AndroidRuntime(10780): at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
E/AndroidRuntime(10780): at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
E/AndroidRuntime(10780): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
E/AndroidRuntime(10780): at android.os.Handler.dispatchMessage(Handler.java:107)
E/AndroidRuntime(10780): at android.os.Looper.loop(Looper.java:214)
E/AndroidRuntime(10780): at android.app.ActivityThread.main(ActivityThread.java:7356)
E/AndroidRuntime(10780): at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(10780): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
E/AndroidRuntime(10780): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
E/AndroidRuntime(10780): Caused by: java.lang.ClassNotFoundException: Didn't find class "com.mindle.inventory.merchant.MainActivity" on path: DexPathList[[zip file "/data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/base.apk"],nativeLibraryDirectories=[/data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/lib/x86, /data/app/com.mindle.inventory.merchant-5Wi-R0Kyd6WC0vFLuCkbvA==/base.apk!/lib/x86, /system/lib, /system/product/lib]]
E/AndroidRuntime(10780): at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:196)
E/AndroidRuntime(10780): at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
E/AndroidRuntime(10780): at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
E/AndroidRuntime(10780): at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
E/AndroidRuntime(10780): at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:41)
E/AndroidRuntime(10780): at android.app.Instrumentation.newActivity(Instrumentation.java:1243)
E/AndroidRuntime(10780): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
E/AndroidRuntime(10780): ... 11 more