No Connected Devices Found in a Flutter App in Android Studio

I read StackOverflow questions including the following ones:

• flutter run: No connected devices

• No connected devices found; please connect a device

• Android studio - flutter: No Connected Devices

BUT, I couldn't solve my issue....... :-(

My Flutter folder structure

• D:\src\flutter

• D:\src\sdk

My Project folder location : D:\__GitHub

Flutter SDK Setup Steps

• Add the following User environment variable

  • Variable Name : ANDROID_SDK_ROOT

  • Variable Value : D:\src\sdk

• Edit the "PATH" variable in the User enviroment variable

  • D:\src\flutter\bin;

  • %ANDROID_SDK_ROOT%\tools;

  • %ANDROID_SDK_ROOT%\platform-tools;

• Run flutter config --android-sdk [sdk location] in the terminal

• Run flutter doctor --android-licenses in the terminal

Launch an AVD in the emulator

• D:\__GitHub\netninja_flutter_app > flutter doctor --android-licenses

  • All SDK package licenses accepted

• D:\__GitHub\netninja_flutter_app > flutter emulator

  • Pixel_2_API_28

• D:\__GitHub\netninja_flutter_app > flutter emulator --launch Pixel_2_API_28

  • 

• When clicked, "Nothing here" shows up.

• When clicked, the following message pops up.

  

• When clicked, the following window shows up.

  

  • I added a virtual device.

Flutter Doctor

• D:\__GitHub\netninja_flutter_app > flutter doctor -v

  • [✓] Flutter (Channel stable, 1.22.2, on Microsoft Windows [Version 6.1.7601], locale en-US)

    • [✓] Flutter version 1.22.2 at D:\src\flutter

    • [✓] Framework revision 84f3d28555 (9 days ago), 2020-10-15 16:26:19 -0700

    • [✓] Engine revision b8752bbfff

    • [✓] Dart version 2.10.2

  • [✓] Android toolchain - develop for Android devices (Android SDK version 30.0.)

    • [✓] Android SDK at D:\src\sdk

    • [✓] Platform android-30, build-tools 30.0.2

    • [✓] ANDROID_SDK_ROOT = D:\src\sdk

    • [✓] Java binary at: D:_programs\Android\jre\bin\java

    • [✓] Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

    • [✓] All Android licenses accepted

  • [✓] Android Studio (version 4.1.0)

    • [!] Flutter plugin not installed; this adds Flutter specific functionality

    • [!] Dart plugin not installed; this adds Dart specific functionality

    • [✓] Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

=> I installed "Dart" & "Flutter" plugins. So, I don't know why the error message says that those plugins are not installed. (This answer says just ignore the error message. So, I did so.)

QUESTION! How could I connect a device to this project?