Fix usage of nullable in gradle 6.0 DomainObjectContext.project

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

@@ -23,7 +23,7 @@ internal fun Project.execWithProgress(description: String, readStdErr: Boolean =
     val stdInPipe = PipedInputStream()
     val exec = services.get(ExecActionFactory::class.java).newExecAction()
     body(exec)
-    return project.operation(description) {
+    return project!!.operation(description) {
         progress(description)
         exec.standardOutput = PipedOutputStream(stdInPipe)
         val outputReaderThread = thread(name = "output reader for [$description]") {