flutter: Multiple projects in this build have project directory

Question

I'm developing a flutter application on Mac, the app runs ok on ios when I try to run it on an Android device, it pops out:

FAILURE: Build failed with an exception.

* What went wrong:
Multiple projects in this build have project directory '/Users/liuqishu/Downloads/scambio/android': [:scambio, :]

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 661ms
Running Gradle task 'assembleRelease'...                            3.5s
Gradle task assembly release failed with exit code 1

Technically, there is only one project in my project directory which is "scambio", but I noticed that in the log [:scambio, :], there is a project with an empty name?

What should I do to remove this empty project from my Gradle?

my build.gradle file:

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
    project.evaluationDependsOn(':app')
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

what's is my project_name/android folder:

- app/build.gradle
- gradle/wrapper
- build.gradle
- gradle.properties
- settings.gradle
- gradlew
- gradlew.bat
- local.properties