'Could not get unknown property 'classesDir' for main classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput.' error - gradle

Question

Since I wanted to examine this source code, I imported it into Android Studio. I got a few other errors before and fixed them. This is annoying, now I have a new error.
I am getting this error:
Build file 'C:\Users\hange\Desktop\libgdx-demo-superjumper-master\desktop\build.gradle' line: 18
A problem occurred evaluating project ':desktop'.
> Could not get unknown property 'classesDir' for main classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput.
* Try:
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My "desktop\build.gradle" file looks like this. I specified line 18 there.
apply plugin: "java"
sourceCompatibility = 1.6
sourceSets.main.java.srcDirs = [ "src/" ]
project.ext.mainClassName = "com.badlogicgames.superjumper.desktop.DesktopLauncher"
project.ext.assetsDir = new File("../android/assets");
task run(dependsOn: classes, type: JavaExec) {
main = project.mainClassName
classpath = sourceSets.main.runtimeClasspath
standardInput = System.in
workingDir = project.assetsDir
ignoreExitValue = true
}
task dist(type: Jar) {
from files(sourceSets.main.output.classesDir) // THIS IS 18th LINE.
from files(sourceSets.main.output.resourcesDir)
from {configurations.compile.collect {zipTree(it)}}
from files(project.assetsDir);
manifest {
attributes 'Main-Class': project.mainClassName
}
}
dist.dependsOn classes
EDIT: My "build.gradle" file looks like this. But I noticed that it can't resolve the json library. Probably the error is here because we are pulling the gradle version from a json file on the internet.
import groovy.json.JsonSlurper // Cannot resolve symbol 'json'
buildscript {
ant.get(src: 'https://libgdx.com/service/versions.json', dest: 'versions.json')
def versionFile = file('versions.json')
def json
if (versionFile.exists()) {
json = new JsonSlurper().parseText(versionFile.text)
} else throw new GradleException("Unable to retrieve latest versions, please check your internet connection")
ext {
gdxVersion = json.libgdxRelease
roboVMVersion = json.robovmVersion
roboVMGradleVersion = json.robovmPluginVersion
androidToolsVersion = json.androidBuildtoolsVersion
androidSDKVersion = json.androidSDKVersion
androidGradleToolsVersion = json.androidGradleToolVersion
gwtVersion = json.gwtVersion
gwtGradleVersion = json.gwtPluginVersion
}
repositories {
mavenLocal()
mavenCentral()
maven { url "https://plugins.gradle.org/m2/" }
maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
jcenter()
google()
}
dependencies {
classpath "org.wisepersist:gwt-gradle-plugin:$gwtGradleVersion"
classpath "com.android.tools.build:gradle:$androidGradleToolsVersion"
classpath "com.mobidevelop.robovm:robovm-gradle-plugin:$roboVMGradleVersion"
}
}

Answer 1

I'm posting the solution in case anyone else has the same problem:
androidToolsVersion, androidSDKVersion, androidGradleToolsVersion, gwtGradleVersion
These 4 variables are pulled from a json file on the internet, but the value pulled from the json file may not always work correctly. Make the variables compatible.
For example this code is using classesDir.The classesDir property was removed in Gradle 5.x.
If you pull to Gradle 4.8.1, you will get the error "Minimum supported Gradle version is 6.5. Current version is 4.8.1.". To fix this error you have to assign 3.2.x value compatible with 4.8.1 gradle version to the androidGradleToolsVersion variable.
androidGradleToolsVersion = "3.2.1"