Fix findUI throwing NullPointerException when extending Vaadin (#13556)

findUI sometimes threw a NPE when the session wasn't set but the UI ID
was. Doesn't occur in normal use, just when doing custom things with
requestStart/requestEnd or runPendingAccessTasks
Change-Id: Id7733567923fa30dcab4946c43b73200c2a0fac2

server/src/com/vaadin/server/VaadinService.java

@@ -982,7 +982,7 @@ public abstract class VaadinService implements Serializable {
         // Get UI id from the request
         String uiIdString = request.getParameter(UIConstants.UI_ID_PARAMETER);
         UI ui = null;
-        if (uiIdString != null) {
+        if (uiIdString != null && session != null) {
             int uiId = Integer.parseInt(uiIdString);
             ui = session.getUIById(uiId);
         }

server/tests/src/com/vaadin/server/VaadinPortletServiceTests.java

@@ -20,8 +20,15 @@ import static org.hamcrest.core.Is.is;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
+import java.util.concurrent.locks.ReentrantLock;
+
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
+import org.mockito.Mockito;
+
+import com.vaadin.shared.ui.ui.UIConstants;
+import com.vaadin.ui.UI;
 
 public class VaadinPortletServiceTests {
 
@@ -188,4 +195,28 @@ public class VaadinPortletServiceTests {
         assertThat(widgetset,
                 is("com.vaadin.portal.gwt.PortalDefaultWidgetSet"));
     }
+
+    @Test
+    public void findUIDoesntThrowNPE() {
+        try {
+            ReentrantLock mockLock = Mockito.mock(ReentrantLock.class);
+            when(mockLock.isHeldByCurrentThread()).thenReturn(true);
+
+            WrappedSession emptyWrappedSession = Mockito
+                    .mock(WrappedSession.class);
+            when(emptyWrappedSession.getAttribute("null.lock")).thenReturn(
+                    mockLock);
+            VaadinRequest requestWithUIIDSet = Mockito
+                    .mock(VaadinRequest.class);
+            when(requestWithUIIDSet.getParameter(UIConstants.UI_ID_PARAMETER))
+                    .thenReturn("1");
+            when(requestWithUIIDSet.getWrappedSession()).thenReturn(
+                    emptyWrappedSession);
+
+            UI ui = sut.findUI(requestWithUIIDSet);
+            Assert.assertNull("Unset session did not return null", ui);
+        } catch (NullPointerException e) {
+            Assert.fail("findUI threw a NullPointerException");
+        }
+    }
 }