[EoC] Check for null sheet contents before announcing close

BUG=888980

Change-Id: Ic1699abfe2b1b911857557ccfdc04f49543c80d5
Reviewed-on: https://chromium-review.googlesource.com/1243046Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593980}

chrome/android/java/src/org/chromium/chrome/browser/widget/bottomsheet/BottomSheet.java

@@ -993,8 +993,12 @@ public class BottomSheet extends FrameLayout
                 mPersistentControlsToken);
 
         for (BottomSheetObserver o : mObservers) o.onSheetClosed(reason);
-        announceForAccessibility(getResources().getString(
-                getCurrentSheetContent().getSheetClosedAccessibilityStringId()));
+        // If the sheet contents are cleared out before #onSheetClosed is called, do not try to
+        // retrieve the accessibility string.
+        if (getCurrentSheetContent() != null) {
+            announceForAccessibility(getResources().getString(
+                    getCurrentSheetContent().getSheetClosedAccessibilityStringId()));
+        }
         clearFocus();
         mActivity.removeViewObscuringAllTabs(this);