modal-dialogs: avoid possible NPE

The NPE occurs with the following sequence:
1. show tab-modal dialog.
2. some portion of the containing view hierarchy is removed from a window.
3. dialog dismissed.
4. animation started in 1 finishes, triggering trying to use an object
   that was set to null.

Step 3 needs to cancel the start animation.

BUG=1127254
TEST=covered by test

Change-Id: I58c80c0a1e8330c0b87b2a56e228d67bc61b2cba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406933
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806696}

chrome/android/javatests/src/org/chromium/chrome/browser/modaldialog/ChromeTabModalPresenterTest.java

@@ -542,6 +542,23 @@ public class ChromeTabModalPresenterTest {
         Assert.assertEquals(BrowserControlsState.BOTH, getBrowserControlsConstraints());
     }
 
+    @Test
+    @SmallTest
+    @Feature({"ModalDialog"})
+    // Ensures an exception isn't thrown when a dialog is dismissed and the View is no longer
+    // attached to a Window. See https://crbug.com/1127254 for the specifics.
+    public void testDismissAfterRemovingView() throws Throwable {
+        PropertyModel dialog1 = createDialog(mActivity, mManager, "1", null);
+        TestThreadUtils.runOnUiThreadBlocking(() -> {
+            mManager.showDialog(dialog1, ModalDialogType.TAB);
+            ViewGroup containerParent = (ViewGroup) mTabModalPresenter.getContainerParentForTest();
+            // This is a bit hacky and intended to correspond to a case where the hosting
+            // ViewGroup is no longer attached to a Window.
+            containerParent.removeAllViews();
+            mManager.dismissAllDialogs(DialogDismissalCause.UNKNOWN);
+        });
+    }
+
     @BrowserControlsState
     private int getBrowserControlsConstraints() {
         return TestThreadUtils.runOnUiThreadBlockingNoException(

components/browser_ui/modaldialog/android/java/src/org/chromium/components/browser_ui/modaldialog/TabModalPresenter.java

@@ -131,6 +131,10 @@ public abstract class TabModalPresenter extends ModalDialogManager.Presenter {
         // has not yet started.
         if (ViewCompat.isAttachedToWindow(mDialogView)) {
             runExitAnimation();
+        } else {
+            // Cancel any existing animations as when the animation completes it may try to make use
+            // of objects that have been set to null.
+            mDialogContainer.animate().cancel();
         }
 
         if (mModelChangeProcessor != null) {