no-jira: add a check if the sampler start too quickly

src/main/java/org/olat/restapi/system/ThreadsWebService.java

@@ -83,6 +83,9 @@ public class ThreadsWebService implements Sampler {
 	
   @Override
 	public synchronized void takeSample() {
-  	CoreSpringFactory.getImpl(ThreadInfosManager.class).takeSample();
+  	ThreadInfosManager manager = CoreSpringFactory.getImpl(ThreadInfosManager.class);
+		if(manager != null) {//check if the manager is loaded
+			manager.takeSample();
+		}
 	}
 }
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