[FLINK-4089][yarn] fix null check in Yarn client

flink-yarn/src/main/java/org/apache/flink/yarn/YarnClusterClient.java

@@ -345,11 +345,13 @@ public class YarnClusterClient extends ClusterClient {
 		if(!isConnected) {
 			throw new IllegalStateException("The cluster has been connected to the ApplicationMaster.");
 		}
+		ApplicationReport lastReport = null;
 		if(pollingRunner == null) {
 			LOG.warn("YarnClusterClient.getApplicationStatus() has been called on an uninitialized cluster." +
 					"The system might be in an erroneous state");
+		} else {
+			lastReport = pollingRunner.getLastReport();
 		}
-		ApplicationReport lastReport = pollingRunner.getLastReport();
 		if(lastReport == null) {
 			LOG.warn("YarnClusterClient.getApplicationStatus() has been called on a cluster that didn't receive a status so far." +
 					"The system might be in an erroneous state");