Handle loop_ NULL case (#9067)

Co-authored-by: Mehrdad <noreply@github.com>

src/ray/object_manager/plasma/store_runner.cc

@@ -136,6 +136,8 @@ void PlasmaStoreRunner::Start() {
 void PlasmaStoreRunner::Stop() {
   if (loop_) {
     loop_->Stop();
+  } else {
+    ARROW_LOG(ERROR) << "Expected loop_ to be non-NULL; this may be a bug";
   }
 }