Fix warnings after KT-48899

All API ToolingModelBuilder is marked as NonNullApi
https://github.com/gradle/gradle/commit/f93bf949486e73655810ab49ad316100b1c2804c#diff-76e89b92025ccef4e2763c5ee8d30bde939c4a9db26bfc913eac88069cbf5d36

libraries/tools/kotlin-allopen/src/main/kotlin/org/jetbrains/kotlin/allopen/gradle/AllOpenSubplugin.kt

@@ -79,7 +79,7 @@ class AllOpenGradleSubplugin @Inject internal constructor(private val registry:
     //region Stub implementation for legacy API, KT-39809
     internal constructor(): this(object : ToolingModelBuilderRegistry {
         override fun register(p0: ToolingModelBuilder) = Unit
-        override fun getBuilder(p0: String): ToolingModelBuilder? = null
+        override fun getBuilder(p0: String): ToolingModelBuilder = error("Method should not be called")
     })
 
     override fun isApplicable(project: Project, task: AbstractCompile): Boolean = true
@@ -92,4 +92,4 @@ class AllOpenGradleSubplugin @Inject internal constructor(private val registry:
                 "Please use an older version of kotlin-allopen or upgrade the Kotlin Gradle plugin version to make them match."
     )
     //endregion
-}
+}

libraries/tools/kotlin-allopen/src/main/kotlin/org/jetbrains/kotlin/allopen/gradle/model/builder/AllOpenModelBuilder.kt

@@ -21,11 +21,9 @@ class AllOpenModelBuilder : ToolingModelBuilder {
         return modelName == AllOpen::class.java.name
     }
 
-    override fun buildAll(modelName: String, project: Project): Any? {
-        if (modelName == AllOpen::class.java.name) {
-            val extension = project.extensions.getByType(AllOpenExtension::class.java)
-            return AllOpenImpl(project.name, extension.myAnnotations, extension.myPresets)
-        }
-        return null
+    override fun buildAll(modelName: String, project: Project): Any {
+        require(canBuild(modelName)) { "buildAll(\"$modelName\") has been called while canBeBuild is false" }
+        val extension = project.extensions.getByType(AllOpenExtension::class.java)
+        return AllOpenImpl(project.name, extension.myAnnotations, extension.myPresets)
     }
-}
+}