[Gradle, MPP] findAndroidTarget: Return null when missing

Prior to this change a 'Collection is empty.' error was reported
when the Android plugin was applied without an Android Kotlin target
being registered.

^KT-41641 Verification Pending

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/KT41641AbsentAndroidTarget.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+@file:Suppress("FunctionName")
+
+package org.jetbrains.kotlin.gradle
+
+import com.android.build.gradle.LibraryExtension
+import org.jetbrains.kotlin.gradle.targets.android.findAndroidTarget
+import org.junit.Test
+import kotlin.test.assertNull
+import kotlin.test.assertSame
+
+class KT41641AbsentAndroidTarget : MultiplatformExtensionTest() {
+    @Test
+    fun `test android plugin without android target`() {
+        project.plugins.apply("kotlin-multiplatform")
+        project.plugins.apply("android-library")
+
+        /* Arbitrary minimal Android setup */
+        val android = project.extensions.getByName("android") as LibraryExtension
+        android.compileSdkVersion(30)
+
+        kotlin.jvm()
+
+        /* Previously failed with 'Collection is empty.' */
+        assertNull(project.findAndroidTarget())
+        assertSame(kotlin.android(), project.findAndroidTarget())
+    }
+}

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/KotlinDependenciesManagement.kt

@@ -29,6 +29,7 @@ import org.jetbrains.kotlin.gradle.plugin.sources.KotlinDependencyScope
 import org.jetbrains.kotlin.gradle.plugin.sources.resolveAllDependsOnSourceSets
 import org.jetbrains.kotlin.gradle.plugin.sources.sourceSetDependencyConfigurationByScope
 import org.jetbrains.kotlin.gradle.plugin.sources.withAllDependsOnSourceSets
+import org.jetbrains.kotlin.gradle.targets.android.findAndroidTarget
 import org.jetbrains.kotlin.gradle.targets.jvm.JvmCompilationsTestRunSource
 import org.jetbrains.kotlin.gradle.tasks.locateTask
 import org.jetbrains.kotlin.gradle.testing.KotlinTaskTestRun
@@ -207,15 +208,6 @@ private fun chooseAndAddStdlibDependency(
     }
 }
 
-private fun Project.findAndroidTarget(): KotlinAndroidTarget? {
-    val kotlinExtension = project.kotlinExtension
-    return when (kotlinExtension) {
-        is KotlinMultiplatformExtension -> kotlinExtension.targets.withType(KotlinAndroidTarget::class.java).single()
-        is KotlinAndroidProjectExtension -> kotlinExtension.target
-        else -> null
-    }
-}
-
 private fun androidMainSourceSetName(project: Project): String {
     val target = project.findAndroidTarget() ?: error("No Android target found")
     return AbstractAndroidProjectHandler.kotlinSourceSetNameForAndroidSourceSet(target, "main")

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/sources/KotlinDependencyScope.kt

@@ -58,4 +58,4 @@ internal fun Project.sourceSetMetadataConfigurationByScope(sourceSet: KotlinSour
             COMPILE_ONLY_SCOPE -> sourceSet.compileOnlyMetadataConfigurationName
             RUNTIME_ONLY_SCOPE -> sourceSet.runtimeOnlyMetadataConfigurationName
         }
-    )
+    )

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/android/FindAndroidTarget.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle.targets.android
+
+import org.gradle.api.Project
+import org.jetbrains.kotlin.gradle.dsl.KotlinAndroidProjectExtension
+import org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
+import org.jetbrains.kotlin.gradle.dsl.kotlinExtension
+import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinAndroidTarget
+
+internal fun Project.findAndroidTarget(): KotlinAndroidTarget? {
+    return when (val kotlinExtension = project.kotlinExtension) {
+        is KotlinMultiplatformExtension -> kotlinExtension.targets.withType(KotlinAndroidTarget::class.java).singleOrNull()
+        is KotlinAndroidProjectExtension -> kotlinExtension.target
+        else -> null
+    }
+}