[FIXED JENKINS-36232] NPE during SCM polling (#2425)

* [FIXED JENKINS-36232] NPE during SCM polling

* add test to trigger the "no veto" code path that has the NPE

* incorporate remark on using @Issue reference

core/src/main/java/jenkins/triggers/SCMTriggerItem.java

@@ -121,7 +121,7 @@ public interface SCMTriggerItem {
             }
             @Override public PollingResult poll(TaskListener listener) {
                 SCMDecisionHandler veto = SCMDecisionHandler.firstShouldPollVeto(asItem());
-                if (!veto.shouldPoll(asItem())) {
+                if (veto != null && !veto.shouldPoll(asItem())) {
                     listener.getLogger().println(Messages.SCMTriggerItem_PollingVetoed(veto));
                     return PollingResult.NO_CHANGES;
                 }

core/src/test/java/jenkins/triggers/SCMTriggerItemTest.java

+package jenkins.triggers;
+
+import static org.mockito.Matchers.any;
+import static org.mockito.Mockito.verify;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.jvnet.hudson.test.Issue;
+import org.mockito.Mockito;
+import org.powermock.api.mockito.PowerMockito;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+import org.powermock.modules.junit4.PowerMockRunner;
+
+import hudson.model.Item;
+import hudson.model.SCMedItem;
+import hudson.model.TaskListener;
+import jenkins.scm.SCMDecisionHandler;
+
+@SuppressWarnings("deprecation")
+@RunWith(PowerMockRunner.class)
+public class SCMTriggerItemTest {
+
+    @Test
+    @Issue("JENKINS-36232")
+    @PrepareForTest(SCMDecisionHandler.class)
+    public void noVetoDelegatesPollingToAnSCMedItem() {
+        // given
+        PowerMockito.mockStatic(SCMDecisionHandler.class);
+        PowerMockito.when(SCMDecisionHandler.firstShouldPollVeto(any(Item.class))).thenReturn(null);
+        SCMedItem scMedItem = Mockito.mock(SCMedItem.class);
+        TaskListener listener = Mockito.mock(TaskListener.class);
+
+        // when
+        SCMTriggerItem.SCMTriggerItems.asSCMTriggerItem(scMedItem).poll(listener);
+
+        // then
+        verify(scMedItem).poll(listener);
+    }
+
+}